ASCL1 and small cell lung carcinoma: The basic helix-loop-helix transcription factor ASCL1 is expressed in ~75% of SCLCs, and genetic profiling distinguishes “classic” ASCL1-positive SCLC from “variant” SCLC expressing neurogenic differentiation factor 1 (NEUROD1), another neuronal master regulator that stimulates a neuroendocrine target gene set distinct from that of ASCL1.