CD90 is a 25-35 kD immunoglobulin superfamily member, also known as Thy-1. It is expressed on hematopoietic stem cells and neurons, all thymocytes, and peripheral T cells, but not on B cells. CD90 is a glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein involved in signal transduction. CD90 is involved in costimulation of lymphocyte proliferation and activation, and hematopoietic stem cell differentiation. CD90 has been shown to interact with CD45. The G7 antibody is able to stimulate T cell proliferation and IL-2 secretion and induce apoptosis of thymocytes and CTL clones.