Total RNA was extracted from 200 000 cells with the RNeasy Mini Kit (Qiagen #74106) with on-column DNase digestion (Qiagen #79254) The RNA-seq library was generated following the manufacturer's instructions for the NEBNext Poly(A) mRNA Magnetic Isolation Module (NEB #E7490S) and the NEBNext Ultra II Directional RNA Library Prep Kit (NEB #E7760L).