CYP1A1 is a gene of the cytochrome P-450 family that has been proposed to be a biomarker of cancer risk. We introduce a polymerase chain reaction-based assay to measure allelic variability in exon 7 of the CYP1A1 gene. This genetic variant is associated with an amino acid change at residue 462 in the aryl hydrocarbon hydroxylase protein product. Previously, measurement of CYP1A1 genotypes at this variant site required two assays, one to detect each allele. By using three primers in a single polymerase chain reaction rather than two primers in each of two polymerase chain reactions, the proposed assay may facilitate population-based study protocols. We estimate the frequency of this polymorphism in a Caucasian population to be 0.03, with an observed heterozygosity of 0.06. We have also confirmed the Mendelian segregation of this polymorphism in four multigeneration Centre d'Etude du Polymorphisme Humain families and have placed this locus in a multilocus linkage map on chromosome 15q. The distribution of this polymorphism was the same in breast cancer cases as in two sets of healthy controls.