This is probably because natural selection is weak on this gene (e.g. s = 2.2% for HLA-DRB1 compared to 10-20% for G6PD/A- [53] and 4-9% for HbC [54], two cases of selection linked to malaria) and would only be detectable by exploring regions where gene flow is reduced (like across geographic barriers) and where differences with neutral markers would be unambiguous. Here, HLA-DRB1 is linked to malaria.