A few years back, when I did the 23andMe test, one of the things I got out of it was that I am heterozygous for a mutation in the PRF1 gene. This gene is responsible for an important immune system protein present in NK and T cells (among others if I'm not mistaken) called Perforin. I am heterozygous for the A91V mutation (rs35947132). The "bad" allele is A. I am therefore AG. Being heterozygous for the A91V mutation isn't necessarily terrible unless there is another mutation in the same gene. Being compound heterozygous with A91V plus some other mutation(s) in the same gene may be very bad. Unfortunately, 23andMe didn't test for these other mutations and all I have is the A91V. In any case, I'm curious about what the status of other people may be. Here's an interesting study, illustrating the usefulness of perforin: "Removal of Toxoplasma gondii Cysts from the Brain by Perforin-Mediated Activity of CD8+ T Cells." Fun fact: I am infected with Toxoplasma gondii. I don't know when I got it but I suspect it was a few decades ago.