S9 Cofactor Kit is a ready-to-use kit containing Buffer Salts, G-6-P, Booster and NADP for preparing the S9 mix in metabolic activation studies in the Ames Test and Micronucleus Test. Liver S9 fractions (the 9000g supernatant of a liver homogenate) contain both microsomal and cytosolic fractions. S9 fraction includes many cytosolic enzymes: N-acetyl transferases, methyltransferases, sulfotransferases, glutathione transferases, aldehyde oxidase, xanthine oxidase, thus both Phase I and II metabolites act simultaneously. Due to the fact that the enzymes in the S9 fractions are diluted significantly, cofactors such as e.g. β-Nicotinamide adenine dinucleotide phosphate-regenerating system (NADPH, Phase I oxidation) are required for activity.

Microsomes do not include the cytosolic enzymes, they include the endoplasmic reticulum subcellular fraction only with cytochrome P450, CYPs, Uridine 5'-diphospho-glucuronosyltransferase (UGT)).