tokens: Using a monoclonal antibody , Lt-4 , directed against human T cell leukemia virus type I ( HTLV-I ) trans-activator ( tax1 ) antigen , we examined the expression of tax1 and related antigens in a variety of T cell lines bearing HTLV-I and related retroviruses , simian T cell leukemia virus type I ( STLV-I ) and HTLV-II , by immunofluorescence and immunoblot assays .