library_construction_protocol: Flash frozen brain hemispheres were homogenized and total RNA ("Input") was isolated from tissue homogenate using Trizol and purified using the Qiagen Rneasy Mini kit. For "RiboTag IP" samples, 12CA5 monoclonal anti-HA antibody (Roche) was coupled to protein G-coated dynabeads (Invitrogen) and incubated with tissue homogenate for 90 min to immunoprecipitate HA-tagged ribosomes and their attached mRNAs. Ribosome-bound mRNAs were isolated using Qiazol and purified using the Qiagen miRNeasy kit. Libraries were prepared according to protovol using the Illumina TruSeq stranded mRNA kit