May be implicated in B-cell differentiation and lymphomagenesis. Monomer or homodimer; disulfide-linked. Seems not to be secreted. Expressed specifically in primary and secondary lymphoid tissues like lymph node, spleen and tonsil. Specifically expressed in B-cells with a high level in normal germinal center B-cells, centroblasts and in a subset of diffuse large B-cell lymphomas. Highly expressed in bone marrow B-cells and weakly in earlier B lineage cells. Expressed in pre-germinal and germinal center B-cells in secondary lymphoid tissues. Also expressed in melanoma and melanocytes. It is uncertain whether Met-1 or Met-8 is the initiator.