ASCL1 and small cell lung carcinoma: In addition to the unique sample cohort, our study presents two aspects of the ITH of SCLC: 1) the heterogeneous expression in malignant cells of genes related to the cell cycle, immunity, hypoxia, EMT, and other hallmarks; 2) the heterogeneous expression of key TFs of SCLC, most commonly ASCL1 and NEUROD1. These results markedly enhanced our understanding of the clinical features of SCLC.