Recently, comprehensive genomic research studies with cell lines, GEMMs, and PDX models have revealed biologically heterogenous SCLC subtypes (Table 3), based on mRNA expression profiles defined by the differential expression of four key transcription regulators: achaete-scute homologue 1 (ASCL1; also known as ASH1), neurogenic differentiation factor 1 (NeuroD1), yes-associated protein 1 (YAP1), and POU class 2 homeobox 3 (POU2F3). Here, ASCL1 is linked to small cell lung carcinoma.