In another cohort of 175 patients with adult-onset HLH, 25 were found to have single heterozygous missense or splice-site mutations in PRF1, MUNC13-4, or STXBP2. Out of the 14 of these patients who had NK cell studies performed, nine had low or absent function. This evidence concerns the gene STXBP2 and hemophagocytic syndrome.