CD90 (Thy-1) is a GPI-anchored molecule and one of the smallest members of the immunoglobulin superfamily consisting of a single V-set domain. The Thy-1.2 alloantigen is expressed on all thymocytes, peripheral T lymphocytes, and some intraepithelial T cells of most mouse strains. It is also expressed in the brain and at varying levels on other non-lymphoid tissues. The 30-H12 monoclonal antibody does not cross react with mouse and rat strains bearing the Thy-1.1 alloantigen (e.g., AKR/J, PL).