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.