B2M and central nervous system leukemia: Our data demonstrated that in patients with non-CNS-involved conditions such as NCNSL, non-CNS leukemia, non-CNS solid tumor, and non-CNS-involved disease, β2-M levels were significantly higher in serum than in CSF, and in contrast to patients with CNS involved by lymphoma or solid tumor, β2-M levels were significantly higher in CSF than in serum (Table 1).