When NMN was added to the culture medium for 1 h, its intracellular contents increased in both cell types (HeLa and U937 cells), indicating that the nucleotide readily permeates the plasma membrane. Accordingly, NAD contents increased in both HeLa and U937 cells exposed to NMN. NMN reduces NAD and ATP depletion in cells undergoing PARP-1 hyperactivation, significantly delaying cell death[3].