CELF1 or HuR was silenced by transfection with a specific siRNA as described previously (Zou et al., 2010 (link); Xiao et al., 2011 (link)). The siRNAs specifically targeting mRNAs encoding CELF1 (siCUGBP1) or HuR (siHuR) and control-siRNA (C-siRNA) were purchased from Santa Cruz. For each 60-mm cell culture dish, 15 μl of the 20 μM stock duplex siCELF1, siHuR, or C-siRNA was used. Forty-eight hours after transfection using Lipofectamine, cells were harvested for analysis.