Cluster of differentiation 138 (CD138), also known as Syndecan-1, is a transmembrane glycoprotein present on the surface of B-cells during late stage differentiation. Anti-CD138 is used to differentiate marginal zone lymphoma from lymphoplasmacytic lymphoma. ALK Large B-Cell Lymphoma (LBCL) commonly stains positively for CD138, but not for CD20 and CD79a. Anti-CD138 reacts positively with HHV8-associated primary effusion lymphoma that lack B-cell markers. CD138 is also a useful marker for identifying and enumerating benign, reactive, or malignant plasma cells from the bone marrow biopsy samples.