Analysis of missense mutations of PRF1 on the expression of perforin. RBL cells were transfected to express perforin bearing each of the missense mutations listed, then sorted with a fluorescence-activated cell sorter (FACS). The mutations were classified according to the HLH patient's genotype: (A) those identified in homozygous patients; (B) mutations identified in compound heterozygotes, where the second allele encoded a frame-shift or premature termination of the protein; and (C) mutations identified in compound heterozygotes with missense mutations in both alleles of PRF1. The Western immunoblots show the relative level of expression of mutated perforin in equivalent numbers of FACS-sorted RBL cells (see “Materials and methods”).