The Thy1 locus determines a surface antigen present on cells of the thymus, a number of mouse leukemias, brain, and in lesser amounts on lymph node and spleen cells. The allele Thy1^a determines an antigenic specificity, Thy-1.1, found in the AKR and RF strains; the allele Thy1^b determines an antigenic specificity, Thy-1.2, found in the C3HeB/Fe and many other strains (J:5243, J:5012, J:4469). The Thy1 antigen is probably present on all T lymphocytes and absent from all B lymphocytes, and it thus serves as a valuable T-cell marker (J:5243). It is very widely used in experiments designed to determine the distribution and function of T-cells. Thy1 specifies a T-cell surface glycoprotein, T25, with a molecular weight of 25 kDa (J:5707). The protein appears to be anchored in the cell membrane by a lipid that is either phosphotidylinositol or closely related to it (J:12016). Thy1 may function in the cell membrane as a signal transduction molecule (J:8333). The Thy1 locus, or possibly a gene closely linked to it, controls quantitative expression of a protein that is the same size as Thy1 and is expressed on thymus and brain but not on lymph node and spleen cells (J:7900).