p19 Gag expression in permanent transfectants. (A) Three 729 stable transfectants were isolated for HTLV-1ΔPBM, HTLV-2+C22, and HTLV-2+C18 as described in “Materials and methods.” Our well-established 729Achneo and 729pH6neo cell clones were used as wtHTLV-1 and wtHTLV-2 stable producer cell lines, respectively. Culture supernatants were harvested at 48 hours and tested for p19 Gag production by ELISA. Clones indicated by asterisks, which produce similar quantities of p19 Gag, were further characterized by Western blot for Tax protein expression using specific antibodies against Tax-1 (B) or Tax-2 (C), and these clones were used in subsequent experiments.