in an experiment, which showed outbreeding mice to exhibit MHC heterozygosity enhanced their health and survival rates against multiple-strain infections. BAFF and autoimmune disease B-cell activating factor (BAFF) is a cytokine encoded by the TNFSF13B gene. A variant of the gene containing a deletion (GCTGT—>A) renders a shorter mRNA transcript that escapes degradation by microRNA, thus increasing expression of BAFF, which consequently up-regulates the humoral immune response. This variant is associated with systemic lupus erythematosus and multiple sclerosis, but heterozygote carriers of the variant have decreased susceptibility to malaria infection. Cardiometabolic disease Variants of the kinase-related MAP3K5 are associated with lifespan. In a study among Japansese-Americans, homozygous individuals have a longer lifespan than heterozygous individuals, indicating a homozygote advantage. See also Balanced polymorphism Hybrid vigour Miscegenation Overdominance Polymorphism (biology) Notes Population genetics Selection