While immunohistochemical staining of PAK1 in breast cancer patient specimens was correlated to tamoxifen resistance [9, 10], possibly related to PAK1-mediated phosphorylation of ERα [11–14], PAK6 binds ERα and inhibits its transcriptional activity, and this PAK6-ERα interaction could be enhanced by tamoxifen [15]. This evidence concerns the gene ESR1 and breast cancer.