RB1 and neoplasm: Primary tumors in the autochthonous SCLC model are initiated by delivering Cre-expressing virus to the murine lung epithelium, leading to the deletion of the Trp53, Rb1, and Pten (collectively known as PRPten) tumor suppressor genes26 and activation of a tdTomato allele that engenders red fluorescence in all tumor cells, including derived CTCs.