Thus, it was found that some mutations in the cytolytic pathway genes, such as perforin 1 (PRF1) and Unc-13 Homolog D (UNC13D) in the HLH family, were linked to MAS and the presence of proinflammatory interleukins, especially IL-6, which depresses cytolytic capacity of the NK cells. Here, PRF1 is linked to macrophage activation syndrome.