CD5 is a glycoprotein monomer with a molecular weight of 67 kDa that belongs to the family of cysteine-rich scavenger receptors (SRCR), a class of extracellular domain-like structures. It has a large cytoplasmic domain that is suitable for signal transduction. CD5 is a T cell marker that also reacts with a number of neoplastic B cells, e.g. in B-cell chronic lymphocytic leukemia (B-CLL), B-cell small lymphocytic lymphoma (B-SLL) and mantle cell lymphoma. CD5 is expressed in T lymphocyte subsets and modulated during cell activation; however, it does not react with granulocytes or monocytes.