The gag and env regions coding for the capsid and envelope proteins, respectively, are the targets for the immune response in cattle and, in BLV and its close relative HTLV (human T-cell leukemia virus), the gag- pol (polymerase)- env segment of the BLV genome is often deleted during the progression of leukemias and lymphomas to advanced stages [5, 26], presumably to escape the host’s immune response. This evidence concerns the gene ERVW-1 and lymphoma.