Using tumor samples as well as tumor derived cell lines, from two different genetically engineered mouse models of tumorigenesis, we find that tumor formation is incompatible with homozygous deletion of both Rock1 and Rock2, since tumors arising in Rock1f/f;Rock2f/f mice always retained either un-recombined Rock1f/for Rock2f/f alleles and showed expression of either ROCK1 or ROCK2, respectively. The gene discussed is ROCK1; the disease is neoplasm.