Furthermore, serum β2-M levels were significantly lower in patients with CNS involvement than in those without lymphoma (CNSL vs. NCNSL: 1.5 mg/L vs. 2.2 mg/L, p < 0.001) or solid tumors (CNS solid tumor vs. non-CNS solid tumor: 1.3 mg/L vs. 2.2 mg/L, p < 0.001), but serum β2-M levels in patients with CNS leukemia were the same as non-CNS leukemia (1.8 mg/L, p = 0.776) (Figs. 2d-f). This evidence concerns the gene B2M and central nervous system leukemia.