A retrospective analysis of all reported cases of FHL associated with bi-allelic PRF1 mutations uncovered a previously unappreciated dichotomy: half of the patients who carried at least one missense mutation in PRF1 presented “acutely,” with a FHL-like syndrome before the age of 12 months, while the remaining 50% had significantly delayed FHL, or presented with other, seemingly unrelated pathologies (23, 37, 45). The gene discussed is PRF1; the disease is hemophagocytic syndrome.