The gene spans 38 kb of DNA, including a 10 kb intron separating the first two exons. CD9 encodes 8 exons, ranging from 63 to 109 base pairs. The coding sequence is highly conserved between species. The promoter contains neither TATA nor CAAT boxes, but does contain several consensus sequences for the binding of transcription factors (GATA, ETS, E2F, NF-kB, AP2) as well as three putative Sp1 binding sites.

The CD9 transcribed RNA has 1246 bases, of which 684 bases (from 112 (Met) to 795 (Val)) encode the protein.

None.

CD9 is a member of the transmembrane 4 superfamily, also called the tetraspanin family. As other tetraspanins, CD9 is a cell-surface protein containing four hydrophobic transmembrane domains (indicated in green) and two extracellular domains (illustrated in violet). CD9 consists of 228 amino acids and weighs 24-27 kDa. CD9 contains four small and highly conserved hydrophobic transmembrane domains (24-27 amino acids); a small N-terminal (11 amino acids) and a C-terminal cytoplasmic (7 amino acids) tails, and a very small intracellular domain (4 amino acids). The remaining part of the protein is composed of two extracellular domains (also called loops; a small one of 20 amino acids and a large one of 83 amino acids). Two disulfide bonds, generated by four well-conserved cysteine residues (C), stabilize the large extracellular domain. CD9 also contains a tetraspanin signature (amino acids 65-89) and a CCG motif (amino acids 152 to 154), but lacks other motifs found on other tetraspanins (DW, PxSc3, Gc4).

CD9 is expressed by a variety of hematopoietic and epithelial cells. It is transiently expressed during development of spinal motoneurons and other fetal nervous system sites, as well as in hematopoietic development. CD9 is glycosylated (the glycosylation site is in the first extracellular loop unlike most glycosylated tetraspanins where the site is located in the second extracellular loop) and acylated. CD9 is also phosphorylated on tyrosine following B-cell activation.
CD9 is up-regulated on activated B and T lymphocytes.

In normal cells, CD9 localizes mainly in the membranes while in cancer cells the protein may also be detected throughout the cytoplasm.

CD9 can interact or form complexes with many other proteins, including other tetraspanins, integrins, EWI molecules, TGF-a, diphtheria toxin receptor, receptor tyrosine kinase, pregnancy specific glycoproteins, and proteins of the immune system such as MHC class II molecules and members of the Ig superfamily. Moreover, probably because of its localization in the cell membrane, CD9 is involved in platelet activation and aggregation, as well as in cell adhesion, spreading, cell motility and tumor metastasis. CD9 also regulates paranodal junction formation, and is required for gamete fusion. Furthermore, CD9 promotes muscle cell fusion and supports myotube maintenance.

Although there are variations in the amino acid sequence in the extracellular loops, the CD9 protein sequence is very well conserved between species (90% between human, mice and rat). CD9 share also some homologies with other tetraspanins, particularly in the transmembrane domains.

Although no genomic CD9 mutation has been reported, in prostate cancer, there is mention of cDNA mutation compatible with an RNA editing mechanism. So far, CD9 has never been implicated in gene fusion that could result in a modified protein.