RB1 and small cell lung carcinoma: For example, the cell‐of‐origin of SCLC was determined by intratracheal injection of cell‐type‐restricted Adeno‐Cre viruses, to inactivate Trp53 and Rb1 in Clara, neuro‐endocrine (NE), and alveolar type 2 (SPC) cells, respectively.