The samples included 13 with active Still’s–MAS, 3 with inactive Still’s–MAS, 18 with pHLH [n = 7 bi-allelic PRF1 mutation, n = 6 bi-allelic UNC13D mutation, n = 3 XIAP deficiency, n = 1 Griscelli syndrome (bi-allelic RAB27A mutation)] and 1 Hermansky–Pudlak syndrome (bi-allelic HPS3 mutation). The gene discussed is UNC13D; the disease is macrophage activation syndrome.