The exact etiology of MAS is not completely understood, but it is often associated with certain triggering factors, including microbial infections (e.g. EBV, CMV), genetic factors (30-40% of MAS/secondary HLH (sHLH) patients have heterozygous defects in perforin-pathway familial HLH genes), and medications (e.g. CAR-T), but also that no associated triggers may be identified (7, 8). The gene discussed is PRF1; the disease is macrophage activation syndrome.