 This includes Human T-lymphotrophic viruses (HTLVs), bovine leukaemia virus (BLV) and simian T-lymphotropic viruses (STLVs).The tax protein produced from HTLV-1 and HTLV-2 have been most extensively studied.The gene for Tax is found within the pX region between the "env" gene and one of the long terminal repeats of the viral genome; which is where the name Tax comes from (transactivator from the X-gene region).