The CD6 (HI210) Antibody recognizes a 100-130 kDa type-I single-chain transmembrane glycoprotein, a receptor for CD166. CD6 is expressed at low levels on immature thymocytes and at high levels on mature thymocytes. CD6 is also expressed at high levels on peripheral blood T cells and at low levels on most peripheral blood B cells. High levels of CD6 are present in T-cell malignancies and certain B-cell malignancies including B-CLL. CD6 not only is expressed on all CD5+ B-CLL, but also on CD5- B-CLL, suggesting a broader distribution of CD6 than CD5 in B-cell malignancies. CD6 has also been found recently to be expressed in a minor subset of myeloid malignancies. CD6 functions as an adhesion molecule as well as a costimulatory molecule for T cell activation.