One copy of the slow caffeine metaboliser SNP, and one copy of the fast version. This makes you more strongly affected by drinking coffee, as caffeine is broken down slower in the liver. Supposedly this increases the risk of heart attacks, although other studies show caffeine is generally good for the heart. It also makes caffeine more effective at preventing Breast Cancer, Alzheimer's Disease, and Parkinson's disease. Too much caffeine will shrink your breasts.