Similarly, mammary tumor samples derived from caveolin-1 knockouts crossed with mouse mammary tumor virus-polyoma middle T antigen [PyMT/Cav-1(-/-)] mice show ERK-1/2 hyperactivation, cyclin D1 up-regulation, and Rb hyperphosphorylation, consistent with dysregulated cell proliferation. The gene discussed is MAPK3; the disease is breast cancer.