(A–D) HCT-116 cells were transfected with either CYP24A1 wild type or a polymorphism and treated with 1 nM of 1,25D. A mammalian two-hybrid assay system was used to analyze enzymatic activity. Western blot analysis using a monoclonal antibody against hCYP24A1 was also used to assess CYP24A1 expression of both endogenous and transfected CYP24A1. (E) The percent reduction in CYP24A1 activity of each SNP relative to the WT CYP24A1 control (both in the presence of 1,25D) was determined by calculating the % reduction of the WT versus the control (second bar minus third bar in each panel), and the % reduction of the SNP versus the control (second bar minus fourth bar in each panel) for the SNPs, and then taking the ratio of %SNP reduction:%WT reduction, in each panel. All CYP24A1 SNPs show reduced activity, with L409S displaying markedly lower activity. Data are representative of three independent experiments with triplicate samples in each group.