Thy-1 or CD90 (Cluster of Differentiation 90) is a 25–37 kDa heavily N-glycosylated，glycophosphatidylinositol (GPI) anchored conserved cell surface protein with a single V-like immunoglobulin domain，originally discovered as a thymocyte antigen. Thy-1can be used as a marker for a variety of stem cells and for the axonal processes of mature neurons. Structural study of Thy-1 lead to the foundation of the Immunoglobulin superfamily，of which it is the smallest member，and led to some of the initial biochemical description and characterization of a vertebrate GPI anchor and also the first demonstration of tissue specific differential glycosylation.