It is assumed that there are three factors instrumental in the development of fully symptomatic MAS: a genetic abnormality (e.g., a mutation in the perforin coding gene PRF1—the first of many discovered to be responsible for MAS), a prolonged inflammation (as in the case of SLE or sJIA), and a trigger, such as an infection [10]. The gene discussed is PRF1; the disease is macrophage activation syndrome.