Mutations in Munc13-4 (UNC13D gene) cause FHL type 3, a form of familial hemophagocytic lymphohistiocytosis characterized by lytic granules that dock but fail to fuse with the plasma membrane at the immune synapse, resulting in patients that present typical features of FHL such as early onset of overwhelming activation of T lymphocytes and macrophages [67] (Table 1). Here, UNC13D is linked to hereditary hemophagocytic lymphohistiocytosis.