UNC13D and hemophagocytic syndrome: HLH is often seen classified into two forms: primary (familial) HLH, which is caused by mutations in genes regulating T-cell and natural killer (NK) cell function (e.g., PRF1, UNC13D, STXBP2), and secondary HLH, which arises as a response to infections, autoimmune diseases, or malignancies (m-HLH).