PIK3CA and neoplasm: Circulating tumor fraction assessment was found to agree well with orthogonal assessment in a sample (n = 19) of plasma assessed for tumor fraction using low-depth, whole-genome sequencing (Pearson r = 0.86; P < .001; Supplementary Figure 2, available online) and tumor fraction correlated with PIK3CA allele fraction (Pearson r = 0.71; P < .001; Supplementary Figure 3, available online) and TP53 allele fraction (Pearson r = 0.79; P < .001; Supplementary Figure 4, available online).