The expression of ASCL1 determines the molecular subtype classification of SCLC into four groups: SCLC-A (ASCL1-dominant), SCLC-N (NEUROD1-dominant), SCLC-P (POU2F3-dominant), and SCLC-I (SCLC-inflamed). The gene discussed is POU2F3; the disease is small cell lung carcinoma.