STXBP1 and hemophagocytic syndrome: In the current report, we studied NK and T-cell function in an individual with late presentation of FHL due to bi-allelic mutations in STXBP2. Intriguingly, in addition to the expected alterations in STXBP2, we also observed a concomitant reduction in the STXBP1 protein, encoded by the paralog, STXBP1. Further analysis of human NK and T cells demonstrated a functional role for STXBP1 in NK and cytotoxic T-cell cytotoxicity and suggests a unique interplay between STXBP/Munc18 proteins regulating the same exocytic event.