library_construction_protocol: Cells were lysed into total lysates and clarified by centrifugation. In the case of YBX1 and ELAVL1: first biotinylated proteins were enriched by immunoprecipitation with anti-biotin antibody and eluted by 2% acetic acid. Eluted lysates were neutralized by NaOH. Then YBX1 and ELAVL1 was enriched by immunoprecipitation with anti-YBX1 and anti-ELAVL1, respectively.  After washing the beads, RNase T1 digestion was performed, 3´ends of the RNA fragments were dephosphorylated using CIP, and RNA was fluorescently 3´end-labeled using T4 Rnl2(1–249)K227Q, followed by dephosporylation with T4 PNK. RNA-protein complexes were separated by SDS-PAGE and excised. After proteinase K treatment, RNA was extracted and 5' end labelled with T4 RNA ligase. Following small RNA cDNA library preparation was performed. In the case of AGO2: first FLAG-AGO2 was immpunoprecipitated, eluted with FLAG-peptide. After washing the beads, RNase T1 digestion was performed, 3´ends of the RNA fragments were dephosphorylated using CIP, and RNA was fluorescently 3´end-labeled using T4 Rnl2(1–249)K227Q, followed by dephosporylation with T4 PNK. RNA-protein complexes were separated by SDS-PAGE and excised and eluted. anti-biotin was immunoprecipitated on the eluted material followed by proteinase K treatment, RNA extraction and 5' end labelling with T4 RNA ligase. Following small RNA cDNA library preparation was performed. Ligation products were reverse transcribed and amplified by PCR.