The highest level of CD81 gene expression is in endometrium (RPKM 241.8), fat (RPKM 237.1), lung (RPKM 191.2) and 24 other tissues in the human body. Proteins encoded by the CD81 gene are important for transduction of signals, cell development, growth and motility (RefSeq NCBI et al., 2019).

Cluster of Differentiation 81 (CD81) is a type of protein, which belongs to tetraspanin family of proteins (Thomas and Mohler, 2011). According to the study, it is an important molecule for the biological system of living organisms that regulates several types of mechanisms in the body (Thomas and Mohler, 2011). The molecule was previously known as TAPA-1 but changed its name to CD81, after the Fifth International Workshop on Human Leukocyte Differentiation Antigens (IUIS/WHO Subcommittee on CD Nomenclature, 1994).
CD81 gene constitutes a protein coding gene which is located in the chromosomal region 11p15.5 of human genome. Using 5A6 (immunoglobulin- γ1), a monoclonal antiproliferative antibody, Oren et al. (1990) performed the first isolation of cDNAs encoding CD81. The chromosomal localization of CD81 gene in the short arm of chromosome 11 was determined by Andria et al. (1991) in Southern blot experiments in which DNA isolated from somatic cell hybrids was used. In order to assign CD81 gene to a specific region on the chromosome 11, Virtaneva et al. (1994) performed experiments by hybridizing CD81 gene to derivative somatic cell hybrid DNAs of human chromosome 11. The exact location of CD81 gene is shown in the figure 1. To date, 17 transcript variants have been found for this gene.
As indicated by Andria et al. (1991), the 5 untranslated region of the CD81 gene is abundant in CpG islands, which is a common feature observed in many housekeeping genes (Andria et al.,1991; Levy et al., 1998). High content of G + C nucleotides in this region correlates with a high susceptibility to methylation events, which can affect the expression of the receptor (Houldsworth et al., 2013). It also contains a TATA box at -25 position and presumably, binding sites for SP1 transcription factor (Maecker et al., 1997). Additionally, it has been reported that the region in which CD81 gene is located constitutes a tumor-suppressor region, which makes a CD81 gene an important candidate for various malignancies.

There are several ongoing studies regarding the CD81 gene due to its transcriptional activity (Hong et al., 2014; Tardif et al., 2005). In an experiment in which promoter analysis was performed, it has been shown that there is an essential role of the SP1 transcription factor in CD81-induced MT1-MMP transcription (Hong et al., 2014). Having CD81 with high MT1-MMP expression levels is of a great importance since it is associated with malignant melanoma development (Hong et al., 2014). The antigen itself is very much vital for the malignancies, primarily due to the location of the CD81 gene in a tumour-suppressor region (Levy et al., 2014). Mature transcript of CD81 is made of eight exons, all of which code information for protein synthesis. The transcript itself is 1,714 bp long.

CD81 antigen, previously known as TAPA1, constitues a membrane protein of 26-kDa molecular weight, which is expressed in many cell types, including hemopoietic, endothelial as well as epithelial cells. It is a founding member of a highly conserved tetraspan family of transmembrane 4 superfamily, also known as tetraspanin family. Members of this family are expressed not only in mammals, but also in all other multicellular organisms, including insects such as Drosophila, plants as well as fungi.
FFIL CD81 is a highly hydrophobic protein which contains a short N- and C-terminal cytoplasmic domains with cytoplasmic cysteins, potential sites of palmitoylation as well as four transmembrane domains (TM 1-4) that hold the protein in a cell membrane. The protein folds itself creating a large extracellular loop (LEL) with CCG motif and two disulfide bridges, small extracellular loop (SEL) and even a smaller loop of intracellular location (Levy, 2014). Unlike many other members of the family, CD81 does not undergo glycosylation. The protein exibits a strong homology with two antigens - CD37 (65% identical) as well as ME491 melanoma-associated antigen (98% identical) also known as CD63 molecule. Originally, CD81 protein was characterized as a target of an antiproliferative antibody that leads to the inhibition of B cell proliferation (Levy, 2014).
C According to the Zimmerman et al. (2016), transmembrane parts of CD81 contain two largely disconnected couples of spirals, covered with a bulky extracellular loop at the exterior membrane leaflet. These two sets of coils converge at the position of internal leaflet to generate an intramembrane abridged with supplementary electron concentration corresponding to a cholesterol molecule in a cavity. Molecular subtleties simulation recognizes a supplementary conformation in which extracellular loop splits considerably from the transmembrane domain. Cholesterol binding appears to modulate CD81 activity in cells, signifying a probable mechanism for regulation of tetraspanin occupation.

On a protein level the expression of CD81 occurs on B-cells, hepatocytes, monocytes/macrophages and on both naive and memory CD4-positive T cells (Takeda et al., 2003; Silvie et al., 2003; van Zelm et al., 2010; Sagi et al., 2012)

CD81 gene can be localised in the tumor-suppressor region where two different isoforms can be found.

CD 81 plays a significant role in several physiological functions such as regulation of cell activation, development, motility and growth (Fagerberg et al., 2014). It is also involved in signal transduction. CD81 encoded glycoprotein associated with the cell surface is known to create a complex with integrins, which helps to promote fusion of muscle cells as well as maintenance of myotubes.
According to Thomas and Mohler (2011), CD81 is expressed on the surface of oocytes and absence of this molecule is associated with a reduced fertility in the mice system due to egg-sperm fusion defects. On the other hand, Kaji et al. (2002) proved that CD81 and CD9 have corresponding utilities in the egg-sperm fusion. The activity of CD9 is reduced by the absence of the CD81 which was observed in the mice model system. There are several reports available in which it has been proven that CD81 actively prevents development of some viral diseases like hepatitis C virus (Nalesnik and Kanto, 2010). Geisert et al. (2002) showed that CD81 is involved in the development of the central nervous system. According to the experiment, in comparison to normal mice, CD81 mutation in the cells of central nervous system results in the development of 30% larger brains.
When CD81 is increased in the body, it may be promoting breast cancer. Scientific study designed by Zhang et al. (2018) on MDA-MB-435S and MDA-MB-231 human breast cancer cell lines showed that when CD81 expression increases as an effect it can promote breast cancer. Malaria parasite Plasmodium uses CD81 membrane protein to attack liver cells in a human body (Bruening et al., 2018). It is also known that CD81 has an ability to bind to cholesterol where the amount of cholesterol controls the expression of CD81 (van Zelm et al., 2010; Vences-Catalán et al., 2015).

Highest homology level of CD81 protein can be found between humans and mice (Boismenu et al., 1996). By analysing the homology between human and mouse variants, it has been shown that the transmembrane domain residues are mostly sequentially conserved. Beside that it has been also proven that extracellular domains significantly differ from each other as presented in a figure 9 (Frolikova et al., 2018). The protein exhibits a strong homology with two antigens - CD37 (65% identical) as well as ME491, melanoma-associated antigen (98% identical) also known as CD63 molecule (Levy et al., 2014).

Mutations associated with CD81 are often caused by viral infections such as hepatitis C. According to the study where the hepatitis C virus envelope protein (E)2 interacts with cellular receptor CD81 it was possible to obtain the modulation of B and T cell function. Despite positive results, the importance of the mutation is unknown even though the mutation was caused by the modulation of these two proteins as well as the effect and the correlation between CD81 and binding regions of HCV Geno/subtypes (Kronenberger B et al., 2004).
Mutations associated with CD81 are often caused by viral infections such as hepatitis C. According to the study where the hepatitis C virus envelope protein (E)2 interacts with cellular receptor CD81 it was possible to obtain the modulation of B and T cell function. Despite positive It is known that CD19 and CD81 depend on each other regardless if the mutation occurs or not. Dependence was described on a group of patients with an increased susceptibility to infection, hypogammaglobulinemia, and normal numbers of mature B cells in blood, but all the patients had a mutation in CD19 in common (van Zelm et al., 2006). Usually, CD81 is expressed on blood cells such as blood lymphocytes, monocytes, basophilic granulocytes, and eosinophils. The absence of CD81 in membrane has never been found in humans before. However, a study performed on B cells from 611 neonatal cord blood samples showed that the absence can occur (Duijts et al., 2009).
Mutations associated with CD81 are often caused by viral infections such as hepatitis C. According to the study where the hepatitis C virus envelope protein (E)2 interacts with cellular receptor CD81 it was possible to obtain the modulation of B and T cell function. Despite positive In a study where a patient had severe nephropathy and profound hypogammaglobulinemia as well as immunodeficiency with decreased memory B cell numbers, with help of DNA sequencing of all 8 exons in which homozygous G>A substitution was found, splice sites were directly identified downstream of exon 6: c.561+1G>A. This kind of exons cause frameshift before the fourth transmembrane domain as well as premature stop (p. Glu188MetfsX13). Because of the mutation, the splice donor site was disrupted and sixth intron in cryptic splice site was used (van Zelm et al., 2010).

CD81 also has an ability to regulate CD19 in B-lymphocytes, which can trigger the progression of hepatitis C virus infection in human cells. Scientific research in which human cells underwent a procedure of immunohistological staining proved that normal hematopoietic tissue resulted in a strong staining for CD81 in a normal germinal centre B cell. Another analytical method called high-dimensional flow cytometry confirmed that among B and T cell subsets, germinal centre B cells had the highest level of CD81 expression (Luo et al., 2010). Moreover, scientific study performed on mice showed that there is a correlation between the amount of CD81 and CD9 presented in somatic cells with successful fertilisation of spices. It is proven that the lack of CD81 and CD9 leads to a lower chance of successful fertilisation. In somatic cells, CD81 and CD9 associate with each other together with other, non-tetraspanin molecules, creating proteolipid complexes. Artificial removal of CD81 gene in mice leads to 40% reduction of female fertility (Rubinstein et al., 2006).

Cluster analysis have found 25 types of genes that were hypermethylated and hypomethylated, respectively, in 20% in all of the studies. One of the most frequently hypermethylated genes was CD81. CD81 has been found in larger amounts as target of polycomb repressive complex 2 in embryonic stem cells in a medical condition called Glioblastoma multiforme, a devastating brain tumor in adults. CD81 is a member of membranal-embedded tetraspanin superfamily and it is usually observed in multiple myelomas. Tetraspanin genes associated with signalling proteins modulate fundamental biological functions and can potentially be involved in CD81 gene as a metastasis suppressor (Martinez et al., 2009).