ASCL1 and small cell lung carcinoma: SCLC can be classified into four subtypes based on transcriptional signatures driven by specific transcription factors: achaete-scute homolog 1 (ASCL1), neurogenic differentiation factor 1 (NEUROD1), yes-associated protein 1 (YAP1), and POU class 2 homeobox 3 (POU2F3).