 Rieder and colleagues have identified 10 non-coding single- nucleotide polymorphisms SNPs in the vitamin K epoxide reductase complex 1 VKORC1 gene that fall into five major haplotypes. Two of these haplotypes, A and B, can be used to determine whether patients require low AA, 2.7 mgday, intermediate AB, 4.9 mgday, or high BB, 6.2 mgday doses of drug. The haplotypes differ according to racial distribution. These haplotypes are responsible for 25 of the variance in dose. Therefore, a combination of cytochrome P450 and VKORC1 genotyping will facilitate optimal warfarin dosing. 