By using mice in which RhoE expression has been ablated by a gene-trap cassette [27], we show here that this protein is dispensable for normal cellular proliferation, but its absence: a) causes lack of contact inhibition; b) cooperates with oncogenes in cellular transformation; c) increases susceptibility to chemical carcinogens in vivo; and d) increases the metastatic potential of cancer cells. This evidence concerns the gene RND3 and cancer.