Genes encoding three PAH-metabolizing enzymes,CYP1A1, GSTT1 and GSTM1, were studied for polymor- phisms. DNA purification was carried out using NucleoSpin®blood XL kits (Machery-Nagel, Germany). DNA con- centration was determined spectrophotometrically. All primers were synthesized by the Bioservice Unit of the National Center for Genetic Engineering and Biotechnology (BIOTEC, Thailand).