Histologic transformation from non-small cell to small cell lung cancer has been reported as a resistance mechanism to targeted therapy in EGFR -mutant and ALK fusion-positive lung cancers. Whether small cell transformation occurs in other oncogene-driven lung cancers remains unknown. Here we analyzed the genomic landscape of two pre-mortem and 11 post-mortem metastatic tumors collected from an advanced, ROS1 fusion-positive lung cancer patient, who had received sequential ROS1 inhibitors. Evidence of small cell transformation was observed in all metastatic sites at autopsy, with inactivation of RB1 and TP53 , and loss of ROS1 fusion expression. Whole-exome sequencing revealed minimal mutational and copy number heterogeneity, suggestive of "hard" clonal sweep. Patient-derived models generated from autopsy retained features consistent with small cell lung cancer and demonstrated resistance to ROS1 inhibitors. This case supports small cell transformation as a recurring resistance mechanism, and underscores the importance of elucidating its biology to expand therapeutic opportunities., Competing Interests: Competing interestsJ.J.L. has served as a compensated consultant or received honorarium from Chugai Pharma, Boehringer-Ingelheim, Pfizer, C4 Therapeutics, Nuvalent, Genentech, and Turning Point Therapeutics; received institutional research funds from Hengrui Therapeutics, Turning Point Therapeutics, Neon Therapeutics, and Novartis; received CME funding from OncLive; and received travel support from Pfizer. D.J. has received scientific advisory board fee from Eisai, EMD Serono, Genentech, Ipsen, Novartis, Guardant, Petra Pharma, Vibliome Therapeutics, and Relay Therapeutics, and institutional research funds from Novartis, Genentech, EMD Serono, Eisai, Takeda, Placon Therapeutics, Takeda, and Amgen. R.B.C. has served as a consultant/advisory board member for Amgen, Array Biopharma, Astex Pharmaceuticals, Avidity Biosciences, BMS, C4 Therapeutics, Chugai, Elicio, Fog Pharma, Fount Therapeutics/Kinnate Biopharma, Genentech, Guardant Health, Ipsen, LOXO, Merrimack, Natera, N-of-one, Novartis, nRichDx, Revolution Medicines, Roche, Roivant, Shionogi, Shire, Spectrum Pharmaceuticals, Symphogen, Taiho, Warp Drive Bio, Zikani Therapeutics; holds equity in Avidity Biosciences, C4 Therapeutics, Fount Therapeutics, nRichDx, and Revolution Medicines; and has received research funding from Asana, AstraZeneca, Lilly, and Sanofi. I.D.-J. has served as a compensated consultant or received honorarium from Boehringer-Ingelheim and Foundation Medicine, and has received research support from Guardant Health, Array, Pfizer, and Genentech. J.F.G. has served as a compensated consultant or received honoraria from Bristol-Myers Squibb, Genentech, Ariad/Takeda, Loxo, Blueprint, Oncorus, Regeneron, Pfizer, Incyte, Novartis, Merck, Agios, Amgen, Array, and Clovis Oncology; research support from Novartis, Genentech/Roche, and Ariad/Takeda; institutional research support from Bristol-Myers Squibb, Tesaro, Moderna, Blueprint, Jounce, Array Biopharma, Merck, Adaptimmune, Novartis, and Alexo; and has an immediate family member who is an employee of Ironwood Pharmaceuticals. A.N.H. has received research support from Pfizer, Novartis, Amgen, Roche/Genentech, Eli Lilly, and Relay Therapeutics. M.M.-K. has served as a compensated consultant for H3 Biomedicine and AstraZeneca; institutional research support from Novartis. A.T.S. has served as a compensated consultant or received honoraria from Pfizer, Novartis, Genentech/Roche, Ariad/Takeda, Ignyta, LOXO, Bayer, Chugai, Blueprint Medicines, KSQ Therapeutics, Daiichi Sankyo, EMD Serono, Taiho Pharmaceutical, TP Therapeutics, Servier, Syros, Foundation Medicine, Guardant, Natera, Achilles, and Archer; has received institutional research funding from Pfizer, Novartis, Roche/Genentech, Ariad, Ignyta, and TP Therapeutics; has received travel support from Pfizer and Genentech; and is currently an employee of Novartis. The remaining authors have no financial interests to declare., (© The Author(s) 2020.)