This classification identifies four main subdivisions based on the level of expression of ASCL1 (achaete-scute homolog 1), classified as SCLC-A; NEUROD1 (neurogenic differentiation factor one), classified as SCLC-N; POU2F3 (pou class 2 homeobox 3), classified as SCLC-P; YAP1 (yes-associated protein 1), classified as SCLC-Y. The gene discussed is ASCL1; the disease is small cell lung carcinoma.