We have previously shown that the AIB1 is required for breast cancer cell proliferation and demonstrated that its transcriptional activity is up-regulated by phosphorylation and down-regulated by sumoylation, and identified PIAS1 as the SUMO E3 ligase that can enhance the sumoylation of AIB1, thereby down-regulating AIB1 transcriptional activity [24], [25]. This evidence concerns the gene NCOA3 and breast cancer.