It has been postulated that a genetic association between sJIA and macrophage activation syndrome (MAS) exists via a mutated perforin gene (PRF1) [52–54] and polymorphism of both MUNC13-4 [55] and interferon regulatory factor 5 (IRF5) [56] genes. The gene discussed is IRF5; the disease is systemic-onset juvenile idiopathic arthritis.