Ac-YVAD-AFC is a substrate whose amino acids YVAD have been shown to be a preferred cleavage site for caspase-1 and -4.[1],[2] Caspase activity can be quantified by fluorescent detection of free AFC (also known as 7-amino-4-trifluoromethylcoumarin), which is excited at 400 nm and emits at 505 nm.
Reference：
[1]. Talanian, R.V., Quinlan, C., Trautz, S., et al. Substrate specificities of caspase family proteases. J. Biol. Chem. 272(15), 9677-9682 (1997).
[2]. Marcelli, M., Cunningham, G.R., Haidacher, S.J., et al. Caspase-7 is activated during lovastatin-induced apoptosis of the prostate cancer cell line LNCaP. Cancer Res. 58(1), 76-83 (1998).