POU2F3 and small cell lung carcinoma: The recently described transcriptional subtypes of SCLC are defined primarily by the selective enrichment of key transcription factors: achaete–scute homolog 1 (ASCL1), neurogenic differentiation factor 1 (NEUROD1), POU domain class 2 transcription factor 3 (POU2F3), and transcriptional coactivator YAP1 [40,41,42].