The occurrence of multiple copies of the ClTW6 gene was investigated in 217 white healthy Spaniards by the combined use ofX&a I and Ew RI restriction fragment length polymorphism (RPLP) analyses. About 3.5% of the alleles yielded an 12.1 kb Ew RI-REP product in combination with the 42 kb X&u I-RPLP product, which is indicative of multiple CTPW6. The prevalence of subjects carry@ multiple CTPW6 was 7%. The 12.1 kb Ew RI-REP product was highly associated (60%) with the presence of the genotype 29wtf42wt, as characterized by mutation-specific polymerase chain reaction and X&u I-RPLP analyses. Six subjects who had multiple CYPW6 had other genotypes, namely, 44wt/42wt (four subjects), 29C/42wt (one subject), and one subject had a 12.1 kb product plus the CYEW6C mutation associated with the 44 kb/42 kb genotype. All subjects identified as carrying multiple CTPW6 had only two CTPW6 copies in the same chromosome and were classified as carriers of the (CYPW6L)), allelic variant. Phenotyping with debrisoquin indicated an increase in the oxidative capacity as a function of the number of functional CTPW6 genes. The metabolic ratio and the 95% confidence limits were as follows: subjects lacking functional genes, 48.8 (95% confidence limits, 14.4 to 79.3); subjects with one Gmctional gene, 2.14 (95% confidence limits, 0.61 to 3.67); subjects with two functional genes, 1.5 (95% confidence limits, 0.88 to 2.14); and subjects with three functional genes, 0.33 (95% confidence limits, 0.22 to 0.45). Our-findings indicate that the prevalence of subjects who are carriers of (~,zWL)), is at least as frequent as the prevalence of poor metabolizers in the population studied. This may have clinical relevance because the duplicated CTPW6 gene encodes an active enzyme, and its presence signiiicantly (p < 0.002) accelerates the oxidative metabolism in the population studied (CL~