Different concentrations of Ac-VEID-Afc substrate (1-150 mM) were digested with 10 nM caspase-6 in caspase cleavage buffer for 1 h in a black 96well plate (Nunc). Fluorescence was measured every 5 min with excitation at 400 nm and emission at 505 nm. The initial linear part of the curve was analyzed.