CDKN1A and breast cancer: This effect may be linked in part to the regulation of expression of key genes by HDAC9 such as the cell cycle inhibitor CDKN1A and the pro-apoptotic genes, BAX and DR4. Interestingly, our data indicated that the effect on cell proliferation was observed when cells were grown as mammospheres and further work will be necessary to define whether HDAC9 expression is deregulated in breast cancer stem cells [39].