PRF1 and systemic-onset juvenile idiopathic arthritis: [20] Both of these syndromes may be classified under the larger umbrella of hemophagocytic diseases, but are distinct in their origins. FLH is most commonly caused by mutations associated with degranulation genes, especially perforin (PRF1) and the syntaxins. The genetic basis of MAS is less well understood but the vast majority of patients first present with sJIA. [28] Indeed, many of the gene changes shared between EBV and DENV were also found in patients with these inflammatory disorders (Figure 4).