Cells were collected and stored in RNALater (# 76106, Quigen). Total RNA was extracted using Rneasy Mini RNA isolation kit (cat# 74134, Qiagen) following Qiagen protocol, with the following modifications: (1) lysis time is increased up to 40 min; (2) on-column DNase I treatment is performed. cDNA libraries were constructed using the NEBNext Ultra II Directional RNA Library Prep Kit for Illumina (cat#E7760S/E7760L) by NEB following the manufacturer's protocol. RNA was fragmented for 5 minutes.