The observations that PAK1 KO inhibited intestinal tumour progression and up-regulated the immune system by increasing the numbers of the splenic lymphocytes in APC∆14/+ mice, but that PAK1 KO did not affect the numbers of splenic lymphocytes in APC+/+ mice, indicate a possible role of PAK1 in the immune response to tumours induced by mutation in the APC gene. The gene discussed is PAK1; the disease is neoplasm.