Glutathione-S-Transferase T1 (GSTT1) and M1 (GSTM1) are key enzymes involved in phase II detoxification. Null genotypes resulting from gene deletions lead to a complete loss of enzymatic activity, reducing the capacity to metabolize xenobiotics and increasing the risk of adverse effects. Although genotype frequencies vary across ethnic groups, data from non-European populations, particularly Andean populations, remain limited. In this cross-sectional study, the frequency of GSTM1 and GSTT1 null genotypes was determined in 206 individuals from Cusco and Junín. Genotyping was performed by PCR using genomic DNA extracted from peripheral blood. The frequency of the GSTM1 null genotype was 49.51%, whereas that of GSTT1 was 25.24%. Combined genotype analysis showed that 63.11% of participants carried at least one null genotype and 11.65% carried both null variants. No significant differences were observed between Cusco and Junín. Compared with previously reported data, these frequencies were similar to those observed in Peruvian coastal and several South American populations. At the intercontinental level, frequencies were comparable to Europe, the Middle East, and Asia, but differed from Sub-Saharan Africa and Native American populations. This first molecular characterization of GSTM1 and GSTT1 null genotypes in Andean populations provides a baseline for pharmacogenetics and precision medicine research in high-altitude settings.

Glutathione-S-Transferase T1 (GSTT1) and M1 (GSTM1) are key enzymes involved in phase II detoxification. Null genotypes resulting from gene deletions are known to cause a complete loss of enzymatic activity and have been associated with altered xenobiotic metabolism in previous studies. Although genotype frequencies vary across ethnic groups, data from non-European populations, particularly Andean populations, remain limited. In this cross-sectional study, the frequency of GSTM1 and GSTT1 null genotypes was determined in 206 individuals from Cusco and Junín. Genotyping was performed by PCR using genomic DNA extracted from peripheral blood. The frequency of the GSTM1 null genotype was 49.51%, whereas that of GSTT1 was 25.24%. Combined genotype analysis showed that 63.11% of participants carried at least one null genotype and 11.65% carried both null variants. No significant differences were observed between Cusco and Junín. Compared with previously reported data, these frequencies were similar to those observed in Peruvian coastal and several South American populations. At the intercontinental level, frequencies were comparable to Europe, the Middle East, and Asia but differed from Sub-Saharan Africa and Native American populations. This first molecular characterization of GSTM1 and GSTT1 null genotypes in Andean populations provides a population-specific genetic baseline for pharmacogenetics and precision medicine research in high-altitude settings.