PAK1 and breast cancer: Similarly, phosphorylation of EBP1 at threonine 261 by p21-regulated serine/threonine kinase, PAK1, in MCF-7 breast cancer cell line results in suppression of EBP1 transcriptional activity, reduction in ErbB2 protein levels and induction of tamoxifen resistance, but a threonine to alanine mutation reverses the effect [34].