Rudin et al. first divided SCLC into four subtypes: SCLC-A, SCLC-N, SCLC-P, and SCLC-Y according to the relative expression of four key transcriptional regulators which include ASCL1, NEUROD1, POU2F3, and YAP1 (yes associated protein 1) [110]. Here, POU2F3 is linked to small cell lung carcinoma.