Mac-3 is a 110 kD type I membrane glycoprotein, also known as CD107b and LAMP-2. It is expressed on lysosomal membranes and the plasma membrane of macrophages and some myeloid cell lines. In the bone marrow, few cells display Mac-3 antigen on the surface, but a large proportion express Mac-3 in the cytoplasm. CD107b has been identified as a ligand for galaptin, an S-type lectin present in the extracellular matrix. Mac-3/CD107b is upregulated in some tumors and increased expression has been correlated with enhanced metastatic potential.