The CD5 antigen is a transmembrane glycoprotein which is expressed on the most mature human T-cells and expression level of CD5 will be increased during T-cell maturation. CD5 is also expressed in a small subset of normal human B-cells as well. CD5 is expressed in most T-cell lymphomas and leukemias and negative expression of the CD5 in T-cell lymphoma indicates a worse prognosis. In B-cell lymphomas eg. small lymphocytic lymphoma, small-cell lymphoma (CD20+), and mantle cell lymphoma are typically CD5 positive and marginal zone lymphoma and follicular lymphoma, are CD5 negative.