Heterozygous mutations in familial HLH-associated genes (such as PRF1, LYST, RAB27A, UNC13D, STXBP2, and STX11) are detected in up to 40% of MAS cases, which is notably higher than the 15% prevalence reported in the general population and among disease control groups. This evidence concerns the gene UNC13D and macrophage activation syndrome.