The Thy1/CD90 cell surface antigen is a GPI-anchored, developmentally regulated protein involved in signaling cascades that mediate neurite outgrowth, T cell activation, tumor suppression, apoptosis, and fibrosis. Thy1/CD90 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. Decreased Thy1/CD90 mRNA and protein expression is associated with the development of epithelial ovarian cancer, suggesting a role as a putative tumor suppressor gene of human ovarian cancer. Research studies indicate that Thy1/CD90 knockout mice have impaired cutaneous immune responses and abnormal retinal development. Thy1/CD90 is epigenetically regulated or deregulated in some disease states, such as pulmonary fibrosis. The potentially reversible hypermethylation of the Thy1/CD90 promoter offers the possibility of novel therapeutic options in this disease.