Stylized liver cell depicting candidate genes involved in the pharmacokinetics of caffeine. The major route of metabolism to paraxanthine is highlighted by a bold arrow, and the key enzyme, CYP1A2, by a star. A fully interactive version is available online at http://www.pharmgkb.org/pathway/PA165884757. AFMU, 5-acetylamino-6-formylamino-3-methyluracil; CYP, cytochrome P450; NAT2, N-acetyl transferase 2; 17U, 1,7-dimethyluric acid; 17X, 1,7-dimethylxanthine; XDH, xanthine dehydrogenase.