A cell surface molecule that is expressed specifically in thymocytes, lymphocytes activated by ConA-stimulation, and peripheral blood lymphocytes. This molecule is involved in signal transmission of the secondary signal (costimulatory signal) essential for the activation of lymphocytes such as T cells and regulates functions of activated lymphocytes such as activated T cells. Disclosed are a polypeptide of the cell surface molecule; a polypeptide fragment thereof; a fusion polypeptide comprising the fragment; a pharmaceutical composition comprising any one of the above; and methods of using the compositions.