To explore the functional consequences of P-Rex1 S313 phosphorylation on its Rac GEF activity we analyzed several breast cancer cell lines for their expression of P-Rex1, with the aim of encountering a cell line with low/undetectable levels of P-Rex1 in which we could analyze the effect of expression of P-Rex1 or P-Rex1-S313A on Rac activity without interference from endogenous P-Rex1. This evidence concerns the gene PREX1 and breast carcinoma.