In this study, we investigated the effects of Rac3 on the aggressiveness of two breast cancer cell lines that show similar levels of Rac3 expression: 1) MDA-MB-231, which is highly aggressive, and for which we had previously established that the increased expression and spontaneous activation of RhoA plays a role in the aggressiveness, and 2) MCF-7, a poorly aggressive and non-invasive cell line. This evidence concerns the gene RAC3 and breast carcinoma.