Cell Research (2006) 16: 25–30. doi:10.1038/sj.cr.7310005; published online 16 January 2006

Due to concerns regarding the overlapping figures in this review that are identical to those contained in a review article that we have co-authored and published earlier, we retract the above paper we published in Cell Research. We apologize for any confusion that may be caused by this matter, although we stand by the scientific contents contained in the Cell Research paper.