These observations have important implications for the interpretation of genotype/phenotype association studies in HH such as in the case of MHC loci associated with the transmission of the phenotype of low CD8+ T lymphocyte numbers where differences occur among HH populations from geographically distant regions (namely in north Portugal, Norway and Alabama, USA) or the association of MHC markers with iron overload. This evidence concerns the gene HLA-C and Tangier disease.