Publisher Summary Expression of CD90 varies considerably among different species. It is expressed on the prothymocyte subpopulation of human leucocytes, on all mouse and rat thymocytes, and on mouse but not rat T cells. Rat bone marrow cells also express CD90. In all three species, the antigen is expressed abundantly in the brain and at varying levels in other nonlymphoid tissues. CD90 is a GPI-anchored molecule consisting of a single IgSF V-set domain. The N-terminus and the site of addition of the GPI anchor of rat CD90 have been confirmed. Although the protein sequence of rat CD90 is invariant, the brain and thymocyte forms of the protein have tissue- and site-specific glycosylation patterns. CD90 associates noncovalently with the Src family tyrosine kinase Fyn. The function of CD90 is unknown. CD90 mAbs can activate T cells in vitro, which is a feature of other GPI-anchored proteins. The tyrosine kinase Fyn, which associates with Thy-1, is selectively required for activation through Thy-1. Thy-1 mAbs can also induce bcl-2-resistant thymocyte apoptosis, and a ligand for Thy-1 on thymic epithelial cells has been proposed.