A mutation in an immune system gene rapidly rose in frequency in Southeast Asia approximately 50,000 years ago because it likely conferred protection against leprosy, which spread to the region from Africa around the same time. The findings, published May 16th in Cell Reports, show that the gene variant, called HLA-B*46:01, encodes a protein that binds to molecules derived from the bacterium that causes leprosy–a chronic infection of the skin and peripheral nerves. This HLS protein then presents these foreign molecules to the immune system, which destroys the infected cells.