A nucleic acid molecule selected from the group consisting of: (a) a nucleic acid molecule encoding a polypeptide consisting of the amino acid sequence as shown in SEQ NO: 4 or 6; (b) a nucleic acid molecule encoding a polypeptide consisting of the amino acid sequence as shown in SEQ ID NO:2, 4, or 6 in which one to ten amino acids are substituted, deleted or added, wherein said polypeptide functions as a cell surface molecule having the following characteristics: (i) expressed in at least thymocytes and mitogen-stimulated lymphoblast cells; (ii) an antibody reactive to said polypeptide induces adhesion between mitogen stimulated-lymphoblast cells; (iii) an antibody reactive to said polypeptide induces proliferation of peripheral blood lymphocytes in the presence of an antibody against CD3; (iv) having a partial amino acid sequence represented by Phe-Asp-Pro-Pro-Pro-Phe in its extracellular region; and (v) having a partial amino acid sequence represented by Tyr-Met-Phe-Met in its cytoplasmic region; (c) a nucleic acid molecule consisting of the nucleotide sequence from 35 to 637 of SEQ ID NO: 4, or from 35 to 685 of SEQ ID NO:6; (d) a nucleic acid molecule encoding an extracellular region of a polypeptide encoded by any one of (a) to (c) or the complementary strand of such nucleic acid molecule, wherein said extracellular region consists of the amino acid sequence from 1 to 141 of SEQ ID NO: 4. (a) a nucleic acid molecule encoding a polypeptide consisting of the amino acid sequence as shown in SEQ NO: 4 or 6; (b) a nucleic acid molecule encoding a polypeptide consisting of the amino acid sequence as shown in SEQ ID NO:2, 4, or 6 in which one to ten amino acids are substituted, deleted or added, wherein said polypeptide functions as a cell surface molecule having the following characteristics: (i) expressed in at least thymocytes and mitogen-stimulated lymphoblast cells; (ii) an antibody reactive to said polypeptide induces adhesion between mitogen stimulated-lymphoblast cells; (iii) an antibody reactive to said polypeptide induces proliferation of peripheral blood lymphocytes in the presence of an antibody against CD3; (iv) having a partial amino acid sequence represented by Phe-Asp-Pro-Pro-Pro-Phe in its extracellular region; and (v) having a partial amino acid sequence represented by Tyr-Met-Phe-Met in its cytoplasmic region; (i) expressed in at least thymocytes and mitogen-stimulated lymphoblast cells; (ii) an antibody reactive to said polypeptide induces adhesion between mitogen stimulated-lymphoblast cells; (iii) an antibody reactive to said polypeptide induces proliferation of peripheral blood lymphocytes in the presence of an antibody against CD3; (iv) having a partial amino acid sequence represented by Phe-Asp-Pro-Pro-Pro-Phe in its extracellular region; and (v) having a partial amino acid sequence represented by Tyr-Met-Phe-Met in its cytoplasmic region; (c) a nucleic acid molecule consisting of the nucleotide sequence from 35 to 637 of SEQ ID NO: 4, or from 35 to 685 of SEQ ID NO:6; (d) a nucleic acid molecule encoding an extracellular region of a polypeptide encoded by any one of (a) to (c) or the complementary strand of such nucleic acid molecule, wherein said extracellular region consists of the amino acid sequence from 1 to 141 of SEQ ID NO: 4.