Several years ago, a project aiming to determine (i) the molecular basis of G-6-PD deficiency, (ii) the distribution of four different mutant alleles previously detected, and (iii) the whole of polymorphic alleles that account for the overall prevalence of G-6-PD deficiency in Mexico was implemented. Nearly 5000 individuals-from the general population and patients with hemolytic anemia-belonging to at least 14 States were screened for G-6-PD deficiency. Seventy-six G-6-PD-deficient subjects were detected and the prevalence of G-6-PD deficiency in 4777 individuals from the general population was 0.71%. Screening for both mutations associated with enzyme deficiency and silent polymorphisms at the G-6-PD gene was performed in the enzyme-deficient individuals by PCR-SSCP combined with restriction enzyme analysis; the silent polymorphisms as well as the nondeficient variant G-6-PD A(376G) were also investigated in 366 G-6-PD normal individuals from the general population. In 88% of the enzyme-deficient individuals it was possible to define the mutation responsible and the type G-6-PD A- variants were the more common in both individuals from the general population and patients with hemolytic anemia. G-6-PD deficiency is heterogeneous at the DNA level in Mexico and up to date 10 different variants-8 in the present project and 2 previously-have been observed: G-6-PD A(-202A/376G), G-6-PD A(-376G/968C), G-6-PD Santamaria(376G/542T), G-6-PD Vanua Lava(383C), G-6-PD Tsukui(del561-563), G-6-PD "Mexico City"(680A), G-6-PD Seattle(844C), G-6PD Guadalajara(1159T),G-6-PD Nashville(1178A), and G-6-PD Union(1360T). The G-6-PD A(-) variants have a relatively homogeneous distribution and along with G-6-PD Santamaria(376G/542T), they account for 82% of the overall prevalence of G-6-PD deficiency in Mexico; all other seven variants represent 9% of the mutant alleles examined, and in the rest of the chromosomes the mutation responsible for the enzyme deficiency remains to be defined. Several of the variants observed in Mexico are common in Africa, South Europe, and Southeast Asia. The prevalence for the variant G-6-PD A(376G) was 1.64%. From 256 possible haplotypes only 14 were observed and haplotype analysis suggests that some of the G-6-PD variants probably were imported to Mexico by population flow from South Europe, Africa, and Southeast Asia. This work (i) identified the G-6-PD variants prevalent in Mexico, (ii) defines their geographical distribution, (iii) contributes to the knowledge of the genetic structure of the Mexican population, and (iv) will facilitate the molecular analysis of the G-6-PD gene in enzyme-deficient Mexican individuals.