Based on data from 3,795 cancer cases and 3,856 controls in the PLCO Trial, we observed significant effect modification of the circulating vitamin D-cancer risk association by Gc isoform (Gc1-2 and Gc2-2 vs. Gc1-1) for bladder cancer, but not for other cancer sites, including colorectal and lung cancer, for which prior studies have noted interactions [15, 18], or for all cancers combined. This evidence concerns the gene GC and cancer.