CD9 is a surface glycoprotein of the tetraspannin family. It is expressed on a variety of cells, including nerve, muscle cells and many cells of hematopoietic origin. CD9 is found to participate in forming a large molecular cell complex with other member proteins, such as MHC class II, CD19, CD5 and other TM4SF molecules. It is reported that CD9 is a marker of marginal zone B cells, B1 cells and plasma cells. The diverse functions of CD9 may largely depend upon its associated molecules on different cells.