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