Thy-1 or CD90 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. It is highly expressed on the surface of adult neurons and is thought to play a role in modulating adhesive and migratory events, such as neurite extension. Thy-1 can be used as a marker for a variety of stem cells and for the axonal processes of mature neurons.