PAK4 and breast cancer: However, given that ERα Ser305 phosphorylation can be mediated by at least also PAK1, a group I PAK member [49, 50], and by PKA [47, 48, 51], the relative importance of these kinases in breast cancer remains unclear, considering also that both PAK1 and PAK4 have been found overexpressed in human breast cancer [10, 21].