STX11 and macrophage activation syndrome: HLH can be primary as a result of an underlying genetic defect in an array of genes such as PRF1, UNC13D, STX11, and other genes that is more commonly diagnosed in infants and children.3 The presentation of HLH is more often secondary, though, and triggered by malignancies (most commonly hematologic), autoimmune diseases (macrophage activation syndrome),4 or infections.