The selection output was amplified by PCR using Q5 Hot Start High-Fidelity 2X Master Mix (NEB, M0494L), amplicons were purified by QIAGEN-MinElute PCR Purification Kit (QIAGEN, 28006) and quantified using a Qubit DNA High-Sensitivity kit (Invitrogen, 32854) before library construction and sequencing. The library preparation was performed by Nextflex Rapid DNA-Seq Kit (BI00 Scientific, 5144-08) following the manufacturer's manual. The libraries were sequenced on the Illumina NovaSeq platform (Illumina, USA) by HitGen.