design: Base editing mutagenesis screen was performed on Spen in haploid mouse embryonic cells. Loss-of-function mutations were selected with 6-Thioguanine. RNA was extracted using Rneasy (QIAGEN), cDNA was synthesized using PrimeScript RT Kit (Takara), the coding region of Spen was amplified using Q5 polymerase (NEB). Libraries were prepped from PCR amplicons using Nextera XT Kit (Illumina) and sequenced on MiSeq system (Illumina).