Reads in FASTQ format were sorted using the 5-base indices for the assignment of individuals. The sequences between the 5-base indices and spacer sequences were obtained and used as molecular barcode tags. Reads with a total length, including the spacer and the sequence following it, >50 bases were aligned to target regions using BWA-MEM , whereas reads with short mapped ends (<40 bases) were discarded. Reads with the same barcode sequences were grouped together; erroneous barcode tags were detected and removed as described previously . Consensus sequences for reads with the same barcode were created using VarScan  as described previously . When more than 85% of reads had the same base at a position, this base was selected as the consensus. For variant detection, we applied a Poisson distribution model to calculate the sequencing error as previously described . We evaluated each target region for the presence of variant(s), setting P = 10−4 as the threshold for detection. We evaluated each base position in codons 12 and 13 of the KRAS gene at the specified threshold value . Common SNP sites and error-prone sites were not considered in our analysis. We used Genome Reference Consortium human genome build 37 (GRCh37/hg19) as the reference genome.