POU2F3 and small cell lung carcinoma: These subtypes are defined by a high expression of four key transcription regulators, three of which are achaete-scute homologue 1 (ASCL1), neurogenic differentiation factor 1 (NEUROD1), POU class 2 homeobox 3 (POU2F3) [9], and they are referred to as SCLC-A, SCLC-N, SCLC-P, respectively.