CD82 antigen, also known as Kai-1, is a widely expressed palmitoylated molecule of the tetraspanin superfamily. KAI1/CD82 is localized on cell membrane and form interactions with other tetraspanins, integrins and chemokines which are respectively responsible for cell migration, adhesion and signaling. CD82/Kai-1 is a component of the promiscuous TIMP-1 interacting protein complex on the cell surface of human adenocarcinoma cells and gives insight into tumorigenic metastatic potential. CD82/Kai-1 suppresses EMT in prostate cancer cells adhered to fibronectin leading to reduced cell migration and invasiveness. CD82/Kai-1 function is important for muscle stem cell function in muscular disorders. Overexpression of CD82/Kai-1 suppresses growth, migration and invasion of oral cancer cells and may be considered as a potential therapeutic target in oral cancer.

The cluster of differentiation (CD) system is commonly used as cell markers in Immunophenotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules associating with the immune function of the cell. There are more than 320 CD unique clusters and subclusters have been identified. Some of the CD molecules serve as receptors or ligands important to the cell through initiating a signal cascade which then alter the behavior of the cell. Some CD proteins do not take part in cell signal process but have other functions such as cell adhesion. CD9 is a member of the transmembrane 4 superfamily, which is also known as the tetraspanin family. CD9 is a cell surface glycoprotein with 4 hydrophobic domains that are described as complex with integrins and other transmembrane 4 superfamily members. It is found expressed on the surface of the exosomes. The protein takes part in cellular signal transduction events and thus play a role in the regulation of cell development and activation, growth and motility. Besides, CD9 seems to be a key role in the egg-sperm fusion during the mammalian fertilization processes. CD9 is found on the membrane of the oocytes and also appears to intervene in maintaining the normal shape of oocyte microvilli.

CD9, also known as Tspan29, 5H9 antigen, Leukocyte antigen MIC3 (MIC3), Motility-related protein, is a multi-pass membrane protein which belongs to the tetraspanin (TM4SF) family or the transmembrane 4 superfamily. CD9 is a cell surface glycoprotein with 4 hydrophobic domains that is described to complex with integrins and other transmembrane 4 superfamily members. The protein takes part in cellular signal transduction events and thus play a role in the regulation of cell development and activation, growth and motility. Besides, CD9 seems to be a key role in the egg-sperm fusion during the mammalian fertilization processes. CD9 also seems to be a key part in the egg-sperm fusion during mammalian fertilization.

CD82, also known as KAI-1, structurally belongs to tetraspanin family while categorised as metastasis suppressor gene on functional grounds. KAI1/CD82 is localized on cell membrane and form interactions with other tetraspanins, integrins and chemokines which are respectively responsible for cell migration, adhesion and signalling. Downregulation of CD82 expression is associated with the advanced stages of many human cancers and correlates with the acquisition of metastatic potential. Recent studies suggest that complex mechanisms underlie CD82 loss of function, including altered transcriptional regulation, splice variant production and post-translational protein modifications, and indicate a central role for CD82 in controlling metastasis as a 'molecular facilitator'. The loss of KAI1/CD82 expression in invasive and metastatic cancers is due to a complex, epigenetic mechanism that probably involves transcription factors such as NFkappaB, p53, and beta-catenin. A loss of KAI1 expression is also associated with the advanced stages of many human malignancies and results in the acquisition of invasive and metastatic capabilities by tumour cells. Thus, KAI1/CD82 is regarded as a wide-spectrum tumor metastasis suppressor.