sentences:  For the next 15 years, it was thought to be the only enzyme capable of adding a chain of ADP-ribose in mammalian cells.During the late 1980s, ADP-ribosyl cyclases, which catalyze the addition of cyclic-ADP-ribose groups to proteins, were discovered.Finally, sirtuins, a family of enzymes that also possess NAD+-dependent deacylation activity, were discovered to also possess mono-ADP-ribosyl transferase activity.