The fact that PAK1 depletion by gene knockout did not affect the immune system of APC+/+ mice, but up-regulated the immune system by increasing the splenic weight and numbers of lymphocytes of APC∆14/+ mice, indicated that the up-regulation of the immune system by PAK1 depletion may be related to tumours induced by APC gene mutation. The gene discussed is APC; the disease is neoplasm.