The median serum β2-m concentration in all 74 patients with HLH was 4.05 mg/L (5th–95th percentile range, 1.9 to 5.7 mg/L), significantly higher than that in the 35 normal volunteers (median, 1.5 mg/L; 5th–95th percentile range 1.1–1.9 mg/L; P < 0.001) (Figure 1). This evidence concerns the gene B2M and hemophagocytic syndrome.