Genome-wide and candidate gene association studies of serum 25(OH)D levels have suggested a role of single nucleotide polymorphisms (SNPs) near the vitamin D binding protein (GC) and CYP2R1. However, the investigators and others have shown that serum 25(OH)D levels are also associated with genetic ancestry. Because existing studies do not adjust for genetic ancestry and the function of these SNPs have yet to be established, it is not known whether these SNPs actually play a role or whether these may be spurious associations due to ancestral background (i.e., population stratification), correlations with truly causal SNPs, or to random chance. In addition, because of the number of SNPs that must be distinguished, genome-wide studies do not capture regions with high homology or sequence repeats, such as the promoter region of CYP24A1 which has a higher guanine-cytosine content. Improved classification of 'low' 25(OH)D levels within the context of a particular genetic background through identification of "rapid" versus "slow" vitamin D metabolizers will likely have important implications for cancer risk.