Cluster of Differentiation 7 (CD7) is an antigen expressed in immature and mature T-lymphocytes, thymocytes, peripheral blood T-cells, natural killer cells, myeloid precursors, fetal liver and bone marrow, a small subpopulation of normal B-cells, and malignant B-cells. The antigen belongs to the immunoglobulin gene superfamily, and plays an important role in T-cell interactions and T-cell/B-cell interactions during early lymphoid development.