CD90/Thy-1, a GPI-anchored molecule and one of the smallest members of the immunoglobulin superfamily of cell surface receptors, consists of a single V-set domain.1-4 It is expressed on thymocytes, peripheral T lymphocytes, some intraepithelial T lymphocytes and neurons of all mouse strains.1,2 MAb G7 stimulates T-cell proliferation and IL-2 secretion, via signalling through the T-cell receptor/CD3 complex.1,5-7 The MAb has also been reported to promote apoptosis of thymocytes and CTL clones,4,7 and to mediate adhesion of thymocytes to thymic stroma.8