Liver tissue fractions offer convenient and enriched sources of native enzymes for metabolic stability screening assays and other applications. Liver fractions are available as single donors or as pooled donors. The pooled donors are formulated to represent an average profile of catalytic activities from many individuals. BD Biosciences 50-donor pool has a gender ratio of approximately 50:50. Equal milligrams of microsomal protein are used from each donor. The CYP activity is not targeted, but based on the law of averages will end up very similar to the existing 20-donor pool. The 50-donor pool is produced in large lot-sizes, which allows easy and safe re-order in the future without need for re-validation.