Can you drink coffee and never feel the effects of caffeine? Or perhaps you are the opposite and feel jittery after just one cup of java. The reasons for that lie in your genetic code. Specifically, in the CYP1A2 gene. CYP1A2 is a gene that encodes a liver enzyme (cytochrome P450 1A2)) involved in caffeine metabolism. Coffee has a stronger effect on people who have at least one C variant of the CYP1A2 gene (rs762551) than on people who have two copies of the A variant. So if