In the last step of duplex sequencing library preparation, Illumina sequencing adapters are added to the tagged double stranded libraries by PCR amplification using primers containing sequencing adapters. During PCR amplification, both complementary strands of DNA are amplified and generate two types of PCR products. Product 1 derives from strand 1's which have a unique tag sequence (called α in Figure 2) next to the Illumina adapter 1 and product 2 has a unique tag (called β in Figure 2) next to the Illumina adapter 1. (In each strand, tag α is the reverse complement of tag β and vice versa). The libraries containing duplex tags and Illumina adapters are sequenced using the Illumina TruSeq system. Reads that are originating from every single strand of DNA form a group of reads (tag families) that share the same tag. The detected families of reads will be used in the next step for analyzing sequencing data.