To date, SCLC subtypes have been associated with the predominant expression of a transcription factor (ASCL-1, NEUROD1, or POU2F3) although it is worth noting that the SCLC-A, -N, -P, and -I subtypes were defined by clusters that arose from NMF clustering and not by the individual factors themselves. This evidence concerns the gene ASCL1 and small cell lung carcinoma.