HDAC6 and breast cancer: In a study of human breast cancer stem cells, a decrease of HDAC6 protein expression in R2N1d (highly tumorigenic M13SV1R2N1 cell lines from a breast epithelial cell type with stem cell phenotypes cells) cells after treatment with the PI3K inhibitor, suggesting that HDAC6 expression could be regulated by PI3K/AKT activity [11].