PRF1 and hemophagocytic syndrome: The first indication that bi-allelic perforin mutations may lead to atypical FHL came from the observation that individuals who inherit two mutant alleles of PRF1, one of which is the common polymorphism C272 > T, encoding the ostensibly conservative Ala91Val (A91V) substitution, presented at an older age than “classic” FHL, and with variable symptoms (13, 38, 39).