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X4 Pharmaceuticals, Inc. – ‘10-K’ for 12/31/23 – ‘EX-101.DEF’

On:  Thursday, 3/21/24, at 5:08pm ET   ·   For:  12/31/23   ·   Accession #:  1628280-24-12578   ·   File #:  1-38295

Previous ‘10-K’:  ‘10-K’ on 3/21/23 for 12/31/22   ·   Latest ‘10-K’:  This Filing   ·   28 References:   

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  As Of               Filer                 Filing    For·On·As Docs:Size             Issuer                      Filing Agent

 3/21/24  X4 Pharmaceuticals, Inc.          10-K       12/31/23   93:9.5M                                   Workiva Inc Wde… FA01/FA

Annual Report   —   Form 10-K   —   SEA’34

Filing Table of Contents

Document/Exhibit                   Description                      Pages   Size 

 1: 10-K        Annual Report                                       HTML   1.97M 
 2: EX-10.37    Material Contract                                   HTML     33K 
 3: EX-23.1     Consent of Expert or Counsel                        HTML     27K 
 7: EX-97.1     Clawback Policy re: Recovery of Erroneously         HTML     49K 
                Awarded Compensation                                             
 4: EX-31.1     Certification -- §302 - SOA'02                      HTML     31K 
 5: EX-31.2     Certification -- §302 - SOA'02                      HTML     31K 
 6: EX-32.1     Certification -- §906 - SOA'02                      HTML     28K 
13: R1          Cover                                               HTML     95K 
14: R2          Audit Information                                   HTML     32K 
15: R3          Consolidated Balance Sheets                         HTML    124K 
16: R4          Consolidated Balance Sheets (Parenthetical)         HTML     37K 
17: R5          Consolidated Statements of Operations and           HTML    104K 
                Comprehensive Loss                                               
18: R6          Consolidated Statements of Convertible Preferred    HTML    117K 
                Stock, Redeemable Common Stock and Stockholders'                 
                Equity (Deficit)                                                 
19: R7          Consolidated Statements of Cash Flows               HTML    120K 
20: R8          Nature of the Business and Basis of Presentation    HTML     37K 
21: R9          Summary of Significant Accounting Policies          HTML     88K 
22: R10         License, Collaboration, and Funding Agreements      HTML     47K 
23: R11         Fair Value of Financial Assets and Liabilities      HTML    129K 
24: R12         Property and Equipment, Net                         HTML     40K 
25: R13         Accrued Expenses                                    HTML     37K 
26: R14         Long-Term Debt                                      HTML     57K 
27: R15         Leases                                              HTML     56K 
28: R16         Commitment and Contingencies                        HTML     33K 
29: R17         Preferred and Common Stock Warrants                 HTML     81K 
30: R18         Common Stock, Redeemable Common Stock and           HTML     48K 
                Convertible Preferred Stock (converted to Common                 
                Stock)                                                           
31: R19         Stock-Based Compensation                            HTML     87K 
32: R20         Income Taxes                                        HTML     89K 
33: R21         Net Loss per Share                                  HTML     52K 
34: R22         Loss on Transfer of Nonfinancial Assets             HTML     31K 
35: R23         Summary of Significant Accounting Policies          HTML    117K 
                (Policies)                                                       
36: R24         Summary of Significant Accounting Policies          HTML     55K 
                (Tables)                                                         
37: R25         Fair Value of Financial Assets and Liabilities      HTML    137K 
                (Tables)                                                         
38: R26         Property and Equipment, Net (Tables)                HTML     39K 
39: R27         Accrued Expenses (Tables)                           HTML     37K 
40: R28         Long-Term Debt (Tables)                             HTML     52K 
41: R29         Leases (Tables)                                     HTML     55K 
42: R30         Preferred and Common Stock Warrants (Tables)        HTML     78K 
43: R31         Stock-Based Compensation (Tables)                   HTML     75K 
44: R32         Income Taxes (Tables)                               HTML     88K 
45: R33         Net Loss per Share (Tables)                         HTML     52K 
46: R34         Nature of the Business and Basis of Presentation    HTML     56K 
                (Details)                                                        
47: R35         Summary of Significant Accounting Policies -        HTML     46K 
                Schedule of Restricted Cash (Details)                            
48: R36         Summary of Significant Accounting Policies -        HTML     39K 
                Schedule of Cash, Cash Equivalents and Restricted                
                Cash (Details)                                                   
49: R37         Summary of Significant Accounting Policies -        HTML     40K 
                Schedule of Property, Plant and Equipment                        
                (Details)                                                        
50: R38         Summary of Significant Policies - Additional        HTML     38K 
                Information (Details)                                            
51: R39         License, Collaboration, and Funding Agreements -    HTML     54K 
                Genzyme Agreement (Details)                                      
52: R40         License, Collaboration, and Funding Agreements -    HTML     39K 
                Georgetown Agreement (Details)                                   
53: R41         License, Collaboration, and Funding Agreements -    HTML     47K 
                Beth Israel Deaconess Medical Center Agreements                  
                (Details)                                                        
54: R42         License, Collaboration, and Funding Agreements -    HTML     41K 
                Research and Development Incentive Program                       
                (Details)                                                        
55: R43         License, Collaboration, and Funding Agreements      HTML     30K 
                (Details)                                                        
56: R44         Fair Value of Financial Assets and Liabilities -    HTML     74K 
                Measured on a Recurring Basis (Details)                          
57: R45         Fair Value of Financial Assets and Liabilities -    HTML     45K 
                Debt Securities Available-for-Sale (Details)                     
58: R46         Fair Value of Financial Assets and Liabilities -    HTML     53K 
                Narrative (Details)                                              
59: R47         Fair Value of Financial Assets and Liabilities      HTML     55K 
                -Aggregate Fair Value of Warrant and Derivative                  
                Liabilities (Details)                                            
60: R48         Fair Value of Financial Assets and Liabilities -    HTML     50K 
                Assumptions (Details)                                            
61: R49         Fair Value of Financial Assets and Liabilities -    HTML     42K 
                Goodwill Rollforward (Details)                                   
62: R50         Property and Equipment, Net (Details)               HTML     49K 
63: R51         Accrued Expenses (Details)                          HTML     39K 
64: R52         Long-Term Debt - Schedule of Long-Term Debt         HTML     41K 
                (Details)                                                        
65: R53         Long-Term Debt - Narrative (Details)                HTML     63K 
66: R54         Long-Term Debt - Hercules Loan Agreements           HTML     35K 
                (Details)                                                        
67: R55         Long-Term Debt - Future Principal Payments          HTML     37K 
                (Details)                                                        
68: R56         Leases (Details)                                    HTML     79K 
69: R57         Leases - Schedule of Components of Lease Expense    HTML     47K 
                (Details)                                                        
70: R58         Leases - Schedule of Maturities of Lease            HTML     44K 
                Liabilities (Detail)                                             
71: R59         Preferred and Common Stock Warrants - Additional    HTML     84K 
                Information (Details)                                            
72: R60         Preferred and Common Stock Warrants - Schedule of   HTML     50K 
                Outstanding Warrants (Detail)                                    
73: R61         Preferred and Common Stock Warrants - Summary of    HTML    103K 
                Outstanding Warrants to Purchase Shares of Common                
                Stock (Details)                                                  
74: R62         Common Stock, Redeemable Common Stock and           HTML    192K 
                Convertible Preferred Stock (converted to Common                 
                Stock) - Additional Information (Details)                        
75: R63         Stock-Based Compensation - Additional Information   HTML     94K 
                (Details)                                                        
76: R64         Stock-Based Compensation - Summary of Stock Option  HTML     79K 
                Activity (Detail)                                                
77: R65         Stock-Based Compensation - Restricted Stock Units   HTML     38K 
                (Details)                                                        
78: R66         Stock-Based Compensation - Summary of Stock-Based   HTML     42K 
                Compensation Expense Classification (Details)                    
79: R67         Stock-Based Compensation - Stock Option Valuation   HTML     52K 
                (Details)                                                        
80: R68         Income Taxes - Schedule of Income Loss Before       HTML     37K 
                Income Taxes (Details)                                           
81: R69         Income Taxes - Reconciliation of Effective Income   HTML     44K 
                Tax Rate (Details)                                               
82: R70         Income Taxes - Deferred Income Tax Asset (Details)  HTML     50K 
83: R71         Income Taxes - Narrative (Details)                  HTML     48K 
84: R72         Income Taxes - Deferred Tax Valuation Allowance     HTML     33K 
                (Details)                                                        
85: R73         Income Taxes (Details)                              HTML     31K 
86: R74         Net Loss per Share - Summary of Basic and Diluted   HTML     65K 
                Net loss per Share Attributable to Common                        
                Stockholders (Details)                                           
87: R75         Net Loss per Share - Additional Information         HTML     39K 
                (Details)                                                        
88: R76         Net Loss per Share - Schedule of Anti-dilutive      HTML     38K 
                Securities Excluded from Computation of Diluted                  
                Net Loss per Share Attributable to Common                        
                Stockholders (Details)                                           
90: XML         IDEA XML File -- Filing Summary                      XML    170K 
93: XML         XBRL Instance -- xfor-20231231_htm                   XML   1.90M 
89: EXCEL       IDEA Workbook of Financial Report Info              XLSX    199K 
 9: EX-101.CAL  XBRL Calculations -- xfor-20231231_cal               XML    190K 
10: EX-101.DEF  XBRL Definitions -- xfor-20231231_def                XML    760K 
11: EX-101.LAB  XBRL Labels -- xfor-20231231_lab                     XML   1.70M 
12: EX-101.PRE  XBRL Presentations -- xfor-20231231_pre              XML   1.22M 
 8: EX-101.SCH  XBRL Schema -- xfor-20231231                         XSD    212K 
91: JSON        XBRL Instance as JSON Data -- MetaLinks              523±   804K 
92: ZIP         XBRL Zipped Folder -- 0001628280-24-012578-xbrl      Zip    586K 


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28 Previous Filings that this Filing References

  As Of               Filer                 Filing    For·On·As Docs:Size             Issuer                      Filing Agent

11/09/23  X4 Pharmaceuticals, Inc.          10-Q        9/30/23   75:6.9M                                   Workiva Inc Wde… FA01/FA
 8/10/23  X4 Pharmaceuticals, Inc.          10-Q        6/30/23   76:7.4M                                   Workiva Inc Wde… FA01/FA
 5/16/23  X4 Pharmaceuticals, Inc.          8-K:1,3,7,8 5/15/23   16:4.6M                                   Workiva Inc Wde… FA01/FA
 3/21/23  X4 Pharmaceuticals, Inc.          10-K       12/31/22   96:34M                                    Workiva Inc Wde… FA01/FA
12/09/22  X4 Pharmaceuticals, Inc.          8-K:1,8    12/06/22   16:953K                                   Workiva Inc Wde… FA01/FA
11/09/22  X4 Pharmaceuticals, Inc.          8-K:5      11/07/22   11:265K                                   Workiva Inc Wde… FA01/FA
 9/01/22  X4 Pharmaceuticals, Inc.          8-K:3,5     9/01/22   11:2.9M                                   Workiva Inc Wde… FA01/FA
 7/01/22  X4 Pharmaceuticals, Inc.          8-K:1,2,3,8 6/30/22   16:1M                                     Workiva Inc Wde… FA01/FA
 3/17/22  X4 Pharmaceuticals, Inc.          10-K       12/31/21  100:10M                                    Workiva Inc Wde… FA01/FA
 3/03/22  X4 Pharmaceuticals, Inc.          8-K:1,3,8   3/03/22   13:744K                                   Workiva Inc Wde… FA01/FA
 1/14/22  X4 Pharmaceuticals, Inc.          8-K:1,9     1/14/22   13:727K                                   Workiva Inc Wde… FA01/FA
11/05/21  X4 Pharmaceuticals, Inc.          8-K:1,3    11/05/21   13:735K                                   Workiva Inc Wde… FA01/FA
 3/19/21  X4 Pharmaceuticals, Inc.          10-K       12/31/20  100:12M                                    Workiva Inc Wde… FA01/FA
 3/19/21  X4 Pharmaceuticals, Inc.          8-K:1,2,3,8 3/18/21   17:1M                                     Workiva Inc Wde… FA01/FA
 8/07/20  X4 Pharmaceuticals, Inc.          S-3                    8:1.3M                                   Donnelley … Solutions/FA
 6/10/20  X4 Pharmaceuticals, Inc.          S-8         6/10/20    5:331K                                   Workiva Inc Wde… FA01/FA
 5/07/20  X4 Pharmaceuticals, Inc.          10-Q        3/31/20   73:7.2M                                   Workiva Inc Wde… FA01/FA
 3/12/20  X4 Pharmaceuticals, Inc.          10-K       12/31/19  115:15M                                    Workiva Inc Wde… FA01/FA
11/27/19  X4 Pharmaceuticals, Inc.          8-K:1,8,9  11/25/19    7:474K                                   Donnelley … Solutions/FA
 6/19/19  X4 Pharmaceuticals, Inc.          8-K:5,9     6/17/19    7:265K                                   Donnelley … Solutions/FA
 5/13/19  X4 Pharmaceuticals, Inc.          8-K/A:9     3/11/19    5:543K                                   Donnelley … Solutions/FA
 4/15/19  X4 Pharmaceuticals, Inc.          8-K:1,8,9   4/12/19    6:455K                                   Donnelley … Solutions/FA
 4/03/19  X4 Pharmaceuticals, Inc.          8-K/A:9     3/11/19    5:912K                                   Donnelley … Solutions/FA
 3/13/19  X4 Pharmaceuticals, Inc.          8-K:2,3,5,8 3/11/19   22:2.6M                                   Donnelley … Solutions/FA
11/27/18  X4 Pharmaceuticals, Inc.          8-K:1,5,7,811/26/18   10:8.9M                                   Donnelley … Solutions/FA
11/20/17  X4 Pharmaceuticals, Inc.          8-K:5,9    11/20/17    3:176K                                   Donnelley … Solutions/FA
11/06/17  X4 Pharmaceuticals, Inc.          S-1/A                  8:4.3M                                   Donnelley … Solutions/FA
10/20/17  X4 Pharmaceuticals, Inc.          S-1                   49:9.2M                                   Donnelley … Solutions/FA
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