Therefore, considering the interpopulational differences in pharmacogene polymorphism frequencies and the potential of ESR1 genotyping for the efficacy and safety of hormonal treatments for breast cancer patients, we aimed to investigate the association between the frequency of ESR1 gene variants and the subcontinental ancestry of different world populations, including Native populations not yet studied in this context to the best of our knowledge. This evidence concerns the gene ESR1 and breast cancer.