Trypsin inhibitor from soybean, Alexa Fluor® 488 conjugate exhibits green-fluorescence (absorption/emission maxima ~495/519 nm). Trypsin inhibitor from soybean (SBTI) is a 21 kD protein that inhibits the catalytic activity of serine proteases. Recently it has been shown to bind to acrosin, an acrosomal serine protease that is associated with binding of spermatozoa and penetration of the zona pellucida.