In 494 patients evaluated within a collaborative European study, the NK degranulation assay clearly differentiated between patients with defects in granule exocytosis and patients with acquired HLH or other hereditary defects, such as perforin, SAP, or XIAP deficiency.21 Once these functional tests suggest a genetic basis for HLH, molecular analysis should be followed, including for parents and siblings. Here, PRF1 is linked to hyperinsulinemic hypoglycemia, familial, 4.