Genotype, expression, and function of perforin in a family affected by FHLH2. (A) The family tree illustrates the parents and siblings of a child with unknown genotype who died of HLH as an infant. All family members are compound heterozygous for 3 different amino acid transitions in perforin. Compared with the parents, sibling 2 has a unique combination of 2 alleles but is healthy. The only remaining combination that is unique is homozygous T435M, the predicted genotype of the child. (B) Flow cytometry was used to detect perforin in NK cells from parents and siblings. Each combination leads to detectable perforin expression, represented by the light gray histogram. The dark gray histogram represents the isotype control. A representative adult control is shown for comparison. The MCF of perforin in the NK cells of the mother and father were less than 2 SDs below normal compared with healthy adults (n = 13 controls, average MCF = 329 ± 98 [2 SD]). (C) The NK function in the family members and 2 healthy adult controls was tested by chromium release assay, using K562 cells as targets and freshly isolated peripheral blood mononuclear cells as effectors. Percentage of chromium release was calculated as (cpm of sample − cpm of blank)/(cpm of maximum − cpm of blank). Shown is the percentage chromium release at 3 effector/target ratios (E/T). The normal range from 10 healthy adults is shown for comparison at each E/T ratio (dashed box; mean ± 2 SD). All persons were noted to have normal NK percentages at the time of the assay.