RAC1 and breast carcinoma: Based upon the observations that (i) TGF-β induces cell cycle arrest in breast carcinoma cells that have retained responsivity to this growth factor and (ii) senescence-associated cell cycle arrest was associated with enhanced TGF-β signaling, we asked whether TGF-β1-arrested malignant breast epithelial cells and senescent HMEC are more motile and, if so, whether the migratory phenotype is controlled by Rac1 and/or Rac1b.