Fig. 1

Error generation in next-generation sequencing data. Normal cells (gray) and cancer cells (blue) shed DNA into the bloodstream. The cancer DNA (blue) contains a mutation (yellow star). The circulating free DNA in the blood is damaged both in vivo and in vitro (green triangle). Errors can be introduced at each PCR duplication during amplification (red circle). Further errors are accumulated during sequencing and mapping (purple square). The final data contains mapped reads, where some mismatches are errors, and others are mutations from tumor cells