Furthermore, it has been shown that TaxD247G but not wild-type Tax protein activates the upstream sequence of the human cellular proto-oncogene c-fos, which contains 2 major cis-acting elements—the CArG box and CRE motif indispensable for the activation of c-fos by the mutant Tax protein—and also increases the levels of endogenous c- fos mRNA considerably in both human and bovine cell lines [8], suggesting that TaxD247G might have the ability to activate the production of virus particles and an enhanced ability to induce leukemia/lymphosarcoma compared with wild-type Tax protein. Here, FOS is linked to leukemia.