For example, the p53‐knockout rats are susceptible to cancer without additional tumour induction and more likely to suffer hepatic hemangiosarcoma with higher morbidity of developing lymphoma than the p53‐/‐ mouse does.6, 7 In addition, the deletion of LDL receptor (Ldlr) in rat was found to have a profound increase in plasma levels of total cholesterol like Ldlr‐/‐ mice. This evidence concerns the gene LDLR and lymphoma.