According to this paper[1], prior work has already established that 42% of the dose variance is controlled by 3 SNPs. It isn't 100%, but 42% is not insignifican

According to this paper[1], prior work has already established that 42% of the dose variance is controlled by 3 SNPs. It isn't 100%, but 42% is not insignificant. [1]: http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Search&db=pubmed&term=19300499 http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Search&db=p... edit: Also on 23andMe's page regarding Wafarin: In January 2010 the FDA updated warfarin's label to say that information on these variants can assist physicians in selecting a starting dose of the drug. The agency also provided initial dosage recommendations for patients with different variant combinations. The FDA does not, however, require that genetic testing be done before prescribing warfarin.