RNA-IP was performed as described (11 , 61 (link)). Briefly, WT and Fmr1 KO NPCs were harvested and homogenized in 1ml of ice-cold lysis buffer (10 mM Hepes [pH 7.4], 200 mM NaCl, 30 mM EDTA, and 0.5% Triton X-100) with 2× complete protease inhibitors (Boehringer-Mannheim). Nuclei and debris were pelleted at 3,000 × g for 10 min; the supernatant was collected and raised to 300 mM NaCl, and clarified at 14,000 × g for 30 min. The resulting supernatant was pre-cleared for 1 h with 100µL recombinant protein G agarose (Invitrogen) (pre-washed with lysis buffer). An aliquot of pre-cleared input was saved for RNA extraction (200 µL) and protein analysis (100 µL). A monoclonal antibody against FMRP (7G1-1, DSHB) or a monoclonal antibody against IgG (5415S, Cell Signaling) was incubated with recombinant protein G dynabeads at 4°C for 2 h and washed 3 times with lysis buffer. RNase Inhibitors (Roche) were added to the remaining lysates. The pre-cleared lysates were immunoprecipitated with antibody-coated recombinant protein G agarose at 4°C for 2 hours. After three washes with lysis buffer, 10% of immunoprecipitate was saved for protein analysis. The remainder was washed one more time and the immunoprecipitate was re-suspended into Trizol (Invitrogen) for RNA isolation.