B2M and plasma cell myeloma: In multiple myeloma, serum β2M level is the main determinant of the International Staging System (ISS, Stage I: β2M < 3.5 mg/dl and albumin > 3.5 g/dl, Stage II: β2M > 3.5 but less than 5.5 mg/dl or β2M < 3.5 mg/l and albumin < 3.5 g/dl, Stage III β2M > 5.5 mg/l).