library_construction_protocol: mTECs were sorted as CD45-, CDR2-, EPCAM+ cells and MHCII (HLA-DR) was used to separate immature mTEClo and mature mTEChi cell populations RNA from sorted mTEChi and mTEClo populations was extracted using the High Pure RNA Isolation Kit (Roche) The libraries were prepared manually as described in Pelechano (2014) Nature Protocols ; yielding libraries only containing molecules derived from 5'Cap RNA.