The murine equivalent of the cDNA encoding the human T11 (CD2) sheep erythrocyte-binding protein has been cloned. It codes for a putative transmembrane protein which is homologous to human T11. In contrast to immunoglobulins whose domains consist of anti-parallel beta sheets, we predict that mouse and human T11 external domains probably belong to the alpha/beta protein folding class. The cytoplasmic region of T11 is a lengthy, proline-rich segment; secondary structural analysis predicts it to have a nonglobular conformation. This elongated tail could allow for interaction with multiple other intracellular proteins and may contain a cation-binding site involved in T lineage activation.