sentences:  During DNA damage or cellular stress PARPs are activated, leading to an increase in the amount of poly-ADP-ribose and a decrease in the amount of NAD+.For over a decade it was thought that PARP1 was the only poly-ADP-ribose polymerase in mammalian cells, therefore this enzyme has been the most studied.Caspases are a family of cysteine proteases that are known to play an essential role in programmed cell death.