library_construction_protocol: Whole tissue RNA was extracted from up to 6 weeks old mouse mammary glands using Precellys hard-tissue beads and TRIzol. RNA was then DNase treated and purified using RNeasy Mini Kit from Qiagen. Before the library preparation for sequencing, 1 μg  of total RNA was treated with NEBNext rRNA Depletion Kit (Human/Mouse/Rat) that removes the ribosomal RNA from the total RNA. The ribosomal depleted RNA was purified with RNeasy mini Elute columns (Qiagen). The absence of rRNA and the quantity of mRNA was measured with Agilent TapeStation 4200. Samples were sequenced with Illumina NextSeq500  (Illumina, San Diego, CA, USA) at the Biomedicum Functional Genomics Unit, University of Helsinki using NEBNext Ultra Directional RNA Library Prep kit. The sequencing was performed as single-end sequencing for read length 75 bp