It may be of importance to identify the human cytochrome P450 enzymes involved in the first step of metabolism of a drug candidate, especially in terms of inter-individual variability. Identification can be performed with a number of microsomal incubations, i.e. incubations with individual human P450 enzymes (cDNA microsomes), incubations with a set of human microsomes (fully characterized in relation to cytochrome P450 probe substrates), but also by incubation with pooled human liver microsomes in the presence of specific inhibitors. Usually, two of these techniques are used for confirmation. Michaelis-Menten kinetics (Km, Vmax) may also be established. For these studies, a radiolabeled substrate is preferred.