Secreted and Transmembrane Protein 1 (SECTM1) is a transmembrane and secreted protein that belongs to the SECTM family. SECTM1 is expressed in a perinuclear Golgi-like pattern. It is detected at the highest levels in peripheral blood leukocytes and breast cancer cell lines. SECTM1 is considered to participate in thymocyte signaling and the hematopoietic/immune system processes. It is reported that SECTM1 is a broadly expressed, IFN-γ-inducible molecule, which functions as a potent costimulatory ligand for T cell activation and is synergistic with anti-CD28.