We analyzed the relationships among genetic defects, CTL activities, and ages at the onset of different FHL subtypes and found that CTL-mediated cytotoxicity was deficient in patients with FHL2 having a PRF1 nonsense mutation and onset was usually in early infancy, while cytotoxicity was low in patients with FHL3 having an UNC13D nonsense mutation and onset also occurred in infancy (Figure 3) (16). Here, PRF1 is linked to hemophagocytic syndrome.