Initially, preclinical studies based on SCLC cell lines, mouse models, and patient-derived xenografts revealed distinct subtypes of SCLC characterized by gene expression programs driven by the transcription factors achaete-scute homolog 1 (ASCL1), neurogenic differentiation factor 1 (NEUROD1), POU class 2 homeobox 3 (POU2F3), and yes-associated protein 1 (YAP1) [22]. The gene discussed is ASCL1; the disease is small cell lung carcinoma.