Exogenous (e.g., environmental) mutagens produce characteristic patterns of mutation. In contrast, endogenous mutation processes likely are associated with an invariant pattern of mutation. Analysis of factor IX gene mutations among large samples of hemophilia B patients from multiple, widely divergent geographic and ethnic populations reveals a remarkably constant mutational pattern, suggesting that the primary germline mutational process results from endogenous processes rather than environmental mutagens. To test this hypothesis further, we have initiated a study of hemophilia B patients from Peru because relatively large populations of AmerIndians can be found with low admixtures of other races. To determine if the factor IX (FIX) germline mutational pattern in AmerIndians differs from the common and putative endogenous pattern, FIX gene mutations were characterized in an initial sample of 10 AmerIndian Peruvian patients with hemophilia B. A minimum of 2.2 kb of the FIX gene was examined by PCR and direct sequencing of all eight exons, the splice junctions, and the promoter region. The pattern of germline mutation in AmerIndians was similar to the pattern of FIX germline mutations from larger U. S. Caucasian or Mexican Hispanic samples (P=0.55 and 0.63, respectively). The similar pattern in this initial sample of the Peru AmerIndian population provides additional support for the inference that the FIX germline mutational pattern results from predominantly endogenous processes rather than exogenous mutagens.