Because Matsuoka found that the 3' LTR of the HTLV-1 provirus is surprisingly hypomethylated, this led him to consider whether such a state would favor an antisense transcript originating from this end of the proviral genome. HIV-1 researchers have long ago discounted the importance of a 3' LTR driven antisense transcript. Yet, in the last decade, independent results from Mesnard (France) and Matsuoka (Japan) have strongly established the existence and biological importance of an HTLV-1 antisense transcript called HBZ. The HBZ RNA encoded by the minus strand of the provirus is capable of encoding a bZIP protein, and Matsuoka and his colleagues have shown that this RNA and its protein are expressed in all Adult T-cell leukemias (ATLs). Importantly, Matsuoka is the first to demonstrate that the expression of HBZ promotes the proliferation of T cells, and he has reported that HBZ expressing transgenic mice develop T cell lymphomas.