The comparison of a SNV ITH, b CNV ITH, c TMB, d average TMB per cluster between pure SCLC (n = 34) and combined lung cancer (n = 6), EGFR mutant (n = 7), and wild type (n = 33), as well as smoking (n = 31) and nonsmoking (n = 8) subgroups. p Value from two-sided Mann–Whitney U test. Boxplots are represented by a centerline, median; box limits, the 25th and 75th percentiles; whiskers extend represent the lower and upper values within 1.5 * inter-quartile range. e, f The proportion of driver genes, passenger genes, and other genes in the trunk and branch. p Value from two-sided Fisher’s exact test. SNV single-nucleotide variant, CNV copy number variation, ITH intratumoral heterogeneity, P-SCLC pure small cell lung cancer, C-SCLC combined small cell lung cancer, TMB tumor mutation burden, TSG tumor suppressor gene.