Genomic DNA was extracted from saliva using Oragene (DNA Genotek) and purified using DNeasy Blood and Tissue Kit (Qiagen) according to manufacturers' instructions. Library preparation followed the manufacturer's instructions (Illumina). Briefly, 8 μg of genomic DNA was  fragmented into about 500bp by Covaris (Covaris, E210). T4 DNA polymerase and Klenow enzyme were then used to convert the overhangs into blunt ends. An 'A' base was added to the 3' end of the blunt phosphorylated DNA fragments, which was ligated with adapters (Illumina) on both ends. The correctly ligated products were purified by 2% agarose gel electrophoresis followed by the QIAquick gel extraction kit (Qiagen). DNA fragments with adapter molecules on both ends were selected and amplified. The PCR (Polymerase Chain Reaction) was performed with two primers that anneal to the ends of the adapters. The number of PCR cycles was minimized to avoid skewing the representation of the library. The PCR products were checked and purified by agarose gel electrophoresis. The fragments size and molar concentration of library were respectively determined by Bioanalyzer (Agilent, 2100) and Real-Time PCR System (ABI, StepOnePlusTM). Qualified library was sequenced on Illumina HiSeq 2000 platform follwing the manufacturer's instructions.