ERBB2 and neoplasm: We considered the possibility of false-negative variant calls, but for both FGFR3 and ERBB2 alterations the majority of discordances did not appear to be due to insufficient tumor fraction or sequencing depth; in these samples, other alterations were identified in both tissue and ctDNA, and sequencing depth across the genes was generally sufficient to detect truncal variants given the corresponding tumor purity (Supplementary Data 8).