Ac-YVAD-pNA is a very selective chromogenic substrate for caspase-1, a key mediator of inflammatory processes formerly known as interleukin (IL)-1-converting enzyme. Ac-YVAD-pNA is used to determine caspase-1 activity. Cleavage of Ac-YVAD-pNA by caspase-1 releases pNA (p-nitroanilide), which can be quantified spectrophotometrically at 405 nm.