ASCL1 and small cell lung carcinoma: While SCLC with variable NE marker expression has been appreciated for 35 years, the leading experts in SCLC biology have recently proposed a classification of four SCLC subtypes based on the expression of key transcription factors: SCLC-A (ASCL1 positive), SCLC-N (NeuroD1 positive), SCLC-P (POU2F3 positive), and SCLC-Y (YAP1 positive).