library_construction_protocol: Between 5 x 10e4 and 5 x 10e5 cells were sorted per sample and total RNA was prepared from cell lysates using the RNeasy Mini Kit (Qiagen) and RNase-Free DNase Set (Qiagen) according to manufacturer's protocol. Full-length cDNA was prepared with the Smart-seq2 protocol (Picelli, S. et al, Nat Protoc. 2014 Jan;9(1):171-81. doi:10.1038/nprot.2014.006). The subsequent library preparation from the amplified cDNA was performed using the Nextera XT DNA library prep kit (Illumina). The libraries were sequenced on a Illumina HiSeq 3000 instrument with 50-bp single-read configuration.