Starting from this, Vastert et al. sequenced PRF1 in 54 s-JIA patients: 11 of 56 (20%) patients were heterozygous for missense mutations in PRF1, and s-JIA patients with history of MAS presented an increased prevalence of A91V mutation (20%) compared with s-JIA patients without history of MAS (9.8%) (137). The gene discussed is PRF1; the disease is macrophage activation syndrome.