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Table of Contents ( Mark One ) ☑ ☐ UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON , D.C. 20549 FORM 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31 , 2020 or TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from to Commission File Number : 001-33958 ΣELLAS LIFE SCIENCES GROUP SELLAS Life Sciences Group , Inc. ( Exact name of registrant as specified in its charter ) Delaware ( State of incorporation ) 20-8099512 ( I.R.S. Employer Identification No. ) Title of Each Class 7 Times Square , Suite 2503 , New York , NY 10036 ( Address of principal executive officers ) ( 646 ) 200-5278 ( Registrant's telephone number , including area code ) Securities registered pursuant to Section ( 12 ( b ) of the Exchange Act : Common Stock , $ 0.0001 Par Value per share Trading Symbol ( s ) SLS Name of Each Exchange on Which Registered The Nasdaq Stock Market LLC Securities registered pursuant to Section ( 12 ( g ) of the Exchange Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes o No x Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Exchange Act . Yes No x O Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the Registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes X No o Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes X No o Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See the definitions of “ large accelerated filer , ” “ accelerated filer , ” “ smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Large accelerated filer Smaller reporting company O ☑ Accelerated filer Emerging growth company ☐ Non - accelerated filer X If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . O