pLKO1 non-target control-shRNA (SHC016), FMR1-targeting shRNA (TRCN0000059758) or FXR1-targeting shRNA (TRCN0000159153) constructs were used. We produced lentiviruses via co-transfection of pCMV-d8.91, pVSV-G and pLKO1 into HEK293T cells using Lipofectamine 3000 (Thermo Fisher Scientific, L3000015). Transduction was carried out according to the standard protocol of the ENCODE consortium62 (link). Briefly, viruses were collected from conditioned media after 48 h co-transfection. Lentivirus-containing media was mixed with the same volume of DMEM media that contain polybrene (8 μg/ml), which was used to infect HeLa, SK-N-BE(2), and KELLY cells. After 24 h, cells were incubated with puromycin (2 μg/ml for HeLa and 1 μg/ml for SK-N-BE(2) and KELLY) for 3–7 days. Knockdown efficiency was evaluated by Western blot. Cells were lysed in RIPA containing complete protease inhibitor cocktail. Cell lysates were then resolved through 8% SDS-PAGE and probed by ADAR1 antibody (Santa Cruz, sc-271854), ADAR2 antibody (Santa Cruz, sc-73409), FMRP antibody (Millipore, MAB2160), FXR1P antibody (Bethyl Laboratories, A303–892A), and FXR2 antibody (Sigma-Aldrich, F1554).