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Onconova Therapeutics, Inc. – ‘10-Q’ for 3/31/20 – ‘EX-101.DEF’

On:  Friday, 5/15/20, at 4:11pm ET   ·   For:  3/31/20   ·   Accession #:  1104659-20-62608   ·   File #:  1-36020

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

 5/15/20  Onconova Therapeutics, Inc.       10-Q        3/31/20   64:5M                                     Toppan Merrill/FA

Quarterly Report   —   Form 10-Q   —   Sect. 13 / 15(d) – SEA’34
Filing Table of Contents

Document/Exhibit                   Description                      Pages   Size 

 1: 10-Q        Quarterly Report                                    HTML    328K 
 2: EX-31.1     Certification -- §302 - SOA'02                      HTML     23K 
 3: EX-31.2     Certification -- §302 - SOA'02                      HTML     23K 
 4: EX-32.1     Certification -- §906 - SOA'02                      HTML     20K 
 5: EX-32.2     Certification -- §906 - SOA'02                      HTML     20K 
30: R1          Document and Entity Information                     HTML     48K 
59: R2          Condensed Consolidated Balance Sheets               HTML     90K 
42: R3          Condensed Consolidated Balance Sheets               HTML     38K 
                (Parenthetical)                                                  
21: R4          Condensed Consolidated Statements of Operations     HTML     48K 
31: R5          Condensed Consolidated Statements of Comprehensive  HTML     35K 
                Loss                                                             
60: R6          Consolidated Statement of Stockholders' Equity      HTML     56K 
                (Deficit)                                                        
43: R7          Condensed Consolidated Statements of Cash Flows     HTML     69K 
22: R8          Nature of Business                                  HTML     41K 
29: R9          Summary of Significant Accounting Policies          HTML     55K 
52: R10         Revenue                                             HTML     50K 
58: R11         Net Loss Per Share of Common Stock                  HTML     33K 
36: R12         Warrants                                            HTML    184K 
14: R13         Balance Sheet Detail                                HTML     82K 
51: R14         Fair Value Measurements                             HTML     88K 
57: R15         Stock-Based Compensation                            HTML    154K 
35: R16         Research Agreements                                 HTML     22K 
13: R17         License and Collaboration Agreements                HTML     32K 
53: R18         Related-Party Transactions                          HTML     24K 
56: R19         Securities Registrations and Sales Agreements       HTML     25K 
34: R20         Summary of Significant Accounting Policies          HTML     65K 
                (Policies)                                                       
24: R21         Revenue (Tables)                                    HTML     51K 
48: R22         Net Loss Per Share of Common Stock (Tables)         HTML     33K 
64: R23         Warrants (Tables)                                   HTML    185K 
33: R24         Balance Sheet Detail (Tables)                       HTML     85K 
23: R25         Fair Value Measurements (Tables)                    HTML     82K 
47: R26         Stock-Based Compensation (Tables)                   HTML    149K 
63: R27         Nature of Business - Liquidity (Details)            HTML     79K 
32: R28         Summary of Significant Accounting Policies -        HTML     21K 
                Segment Information (Details)                                    
25: R29         Summary of Significant Accounting Policies -        HTML     29K 
                Leases (Details)                                                 
16: R30         Revenue (Details)                                   HTML     38K 
37: R31         Net Loss Per Share of Common Stock (Details)        HTML     27K 
54: R32         Warrants (Details)                                  HTML    131K 
49: R33         Balance Sheet Detail - Prepaid Expenses and Other   HTML     33K 
                Current Assets (Details)                                         
17: R34         Balance Sheet Detail Property and Equipment         HTML     28K 
                (Details)                                                        
38: R35         Balance Sheet Detail - Accrued Expenses and Other   HTML     29K 
                Current Liabilities (Details)                                    
55: R36         Fair Value Measurements (Details)                   HTML     34K 
50: R37         Fair Value Measurements - Rights Offering           HTML     42K 
                (Details)                                                        
15: R38         Fair Value Measurements - Weighted-average          HTML     30K 
                assumptions (Details)                                            
39: R39         Fair Value Measurements - Fair Value Hierarchy      HTML     30K 
                Table (Details)                                                  
19: R40         Fair Value Measurements - Transfers (Details)       HTML     28K 
26: R41         Stock-Based Compensation (Details)                  HTML     41K 
61: R42         Stock-Based Compensation - Expense (Details)        HTML     29K 
44: R43         Stock-Based Compensation - Activity (Details)       HTML     78K 
20: R44         Stock-Based Compensation - Outstanding and          HTML     62K 
                Exercisable (Details)                                            
27: R45         Stock-Based Compensation - Unrecognized             HTML     25K 
                Compensation Expense (Details)                                   
62: R46         Stock-Based Compensation - Fair Value Assumptions   HTML     37K 
                (Details)                                                        
45: R47         Research Agreements (Details)                       HTML     23K 
18: R48         License and Collaboration Agreements (Details)      HTML     68K 
28: R49         Related-Party Transactions (Details)                HTML     30K 
41: R50         Securities Registrations and Sales Agreements -     HTML     49K 
                Underwriting agreement with HCW (Details)                        
40: XML         IDEA XML File -- Filing Summary                      XML    111K 
12: EXCEL       IDEA Workbook of Financial Reports                  XLSX     65K 
 6: EX-101.INS  XBRL Instance -- ontx-20200331                       XML   1.58M 
 8: EX-101.CAL  XBRL Calculations -- ontx-20200331_cal               XML     98K 
 9: EX-101.DEF  XBRL Definitions -- ontx-20200331_def                XML    385K 
10: EX-101.LAB  XBRL Labels -- ontx-20200331_lab                     XML    953K 
11: EX-101.PRE  XBRL Presentations -- ontx-20200331_pre              XML    672K 
 7: EX-101.SCH  XBRL Schema -- ontx-20200331                         XSD    137K 
46: ZIP         XBRL Zipped Folder -- 0001104659-20-062608-xbrl      Zip    113K 


‘EX-101.DEF’   —   XBRL Definitions — ontx-20200331_def


This Exhibit is an XBRL XML File.


                                                                                                                                                                                
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3 Subsequent Filings that Reference this Filing

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

 2/11/21  Onconova Therapeutics, Inc.       424B5                  1:429K                                   Toppan Merrill/FA
 2/10/21  Onconova Therapeutics, Inc.       424B5                  1:431K                                   Toppan Merrill/FA
 1/08/21  Onconova Therapeutics, Inc.       424B5                  1:292K                                   Toppan Merrill/FA
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