An experiment for the upper-division undergraduate biochemistry laboratory is described in which students investigate the influence of genetic variations of cytochrome P450 1A2 on drug metabolism, using caffeine as a model compound. Saliva samples from human subjects are characterized for a single-nucleotide polymorphism in the CYP1A2 gene (genotyping) and for the rate of caffeine clearance (phenotyping). This experiment has clinical significance in the area of personalized medicine.