library_construction_protocol: Cells were sorted directly into extraction buffer of PicoPure RNA Isolation Kit (Applied Biosystems) and RNA was extracted according to manufacturer's protocol with on-column DNase I treatment (Qiagen 79254). RNA-seq libraries were generated by the NEBNext Ultra II Directional RNA Library Prep Kit for Illumina (NEB, E7760S) and the NEBNext Poly(A) mRNA Magnetic Isolation Module (NEB, E7490) according to manufacturer's protocol. Quantified and validated libraries were subjected to single-end sequencing on HiSeq 4000 sequencing system (Illumina).