FHL2 and hemophagocytic syndrome: To determine if hypogammaglobulinemia is a feature of other forms of FHL, we analyzed IgM and IgG levels in sera of FHL-2 (PRF1 deficient), FHL-3 (UNC13D deficient), and FHL-4 (STX11 deficient) patients with acute HLH (patient information: Table S1).