library_construction_protocol: RNA was isolated from 1-3x105 sorted cells using RNeasy plus Micro Kits (Qiagen), according to the manufacturer's instructions. RNA concentration and integrity number were quantified by RNA quality control (QC) analysis conducted by Stanford Protein and Nucleic Acid (PAN) Facility. About 30ng of RNA per sample was used for preparing sequence library. After the ribo-minus RNA was fragmented, cDNA was synthesized using superscript double strand cDNA synthesis kit (Invitrogen). The cDNA samples were then end repaired, ligated with illumine adaptor using NEB next ultra II end repair/dA-tailing and ligation module (New England Biolabs), and amplified with illumine sequencing primer.