Titration of DNA extracted from the bladder cancer cell lines MGH-U3 (FGFR3 mutant) and SW-780 (TERT promoter mutant) into wtDNA demonstrated that the percentage of mutant reads reliably reflected the percentage of mutant DNA down to levels below the 2.5% threshold used here (S2 Fig). Here, FGFR3 is linked to urinary bladder cancer.