PRF1 and Niemann-Pick disease, type C1: Moreover, we recently discovered that another sphingolipidosis, Niemann-Pick disease type C1, NPC1, leads to severely impaired CTL function due to the suppression of perforin activity despite normal expression levels; this was hypothesized to be due to abnormal accumulation of cholesterol and sphingomyelin in the cytotoxic granules that store perforin (20).