Thy-1orCD90(Cluster ofDifferentiation90) is a 25–37 kDaheavily N-glycosylated,glycophosphatidylinositol(GPI) anchored conserved cell surfaceproteinwith a single V-likeimmunoglobulin domain, originally discovered as athymocyteantigen. Thy-1 can be used as a marker for a variety ofstem cellsand for theaxonal processesof matureneurons. Structural study of Thy-1 lead to the foundation of theImmunoglobulin 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.