# The Less Known Cyclins--Uncovered
## 4 Cyclin G  
The G type cyclin may be divided into two types [32]. The first is cyclin G1 encoded by the CCNG1 gene, with molecular weight 34 kDa [33]. The second is cyclin G2 encoded by CCNG2, with a molecular weight of 40 kDa [31]. G-type cyclins are able to interact with several proteins from the cyclin-dependent kinase family, such as Cdk2 and Cdk4, but also with serine/threonine-protein phosphatase PP2A-2 catalytic (PP2A) and regulatory subunits (WDB). G-type cyclins (G1 and G2) are crucial factors for tissue differentiation [32]. Cyclin G1 was first identified as one of the first targets of p53 and interacts with several cell cycle regulators, including Mdm2, ADP ribosylation factors (ARF) and Rb [34; 35]. In normal cells, cyclin G is involved in the regulation of transcription factor with the properties of a tumor suppressor (p53). [PERSON] et al., 2004 [PERSON] et al., 2004 [PERSON] et al. (2004) examined MEF G \(-/-\)and MEF G \(+/+\) cells. They treated both cell lines with DNA damaging factors: MMC and y-irradiation. The effect of DNA damage was the activation of the p53 protein, which was dependent on ataxia-telangiectasia mutated (ATM) protein. Increased levels of p53 have been observed in G \(-/-\) cells, resulting in cell cycle arrest. In contrast, in G \(+/+\) cells the expression level of p53 was lower [PERSON] et al. (2004). Furthermore, cyclin G plays an important role in regulating the p53-Mdm2 pathway in normal cells [PERSON] et al. (2004). [PERSON] in his research used MEF cells. In the MEF G \(-/-\) line, the author observed that Mdm2 is less efficiently phosphorylated by PP2A, which leads to a lower level of stabilization of the p53 protein. In contrast, the MEF G \(+/+\) line restored normal PP2A activity, resulting in better performance in Mdm2 phosphorylation, and thus p53 degradation [PERSON] et al. (2004). In cancer cells, cyclin G plays a dual role in the tumorigenesis process. On the one hand, overexpression of cyclin G leads to the growth of cancer cells; on the other, its presence affects the suppression of cell proliferation. Elevated expression of cyclin G was observed in colorectal cancer [38]. [PERSON] et al., 2003 [PERSON] et al. (2013) studied 90 cases of human rectal and colon cancer. The authors showed that about 90% of the tissues examined were characterized by overexpression of cyclin G. However, the authors did not observe a correlation between pathological and clinical features and elevated expression of cyclin G [38]. Moreover, overexpression of cyclin G was also observed in breast and prostate cancers [39]. [PERSON] et al., 1999 [39] examined breast cancer cell lines: MDAMB435, MDAMB436, HS578t, MCF7, T47D; prostate cancer cell lines: DU145, LNCaP, PC3, ND1; and breast epithelial line (hNMEC) and normal prostate cell lines (NPrEC-1 and NPrEC-2). Using comparative analysis, the authors showed that increased levels of cyclin G expression occur in both types of cancers. In contrast, normal cells of the breast and prostate epithelium showed a lower level of expression of the tested protein. To confirm the conclusions, the authors used immunohistochemistry to analyze the distribution of cyclin G in normal (10 biopsies) and cancer (30 biopsies) tissues of the breast. The results confirmed the observations made during in vitro studies. Cyclin G overexpression has also been shown in human osteosarcoma [39; 40]. [PERSON] et al., 1995 [41] studied the expression of cyclin G in the MG-63 line. The authors showed that cyclin G is overexpressed in this type of cancer. Furthermore, this protein is crucial for the growth and survival of osteosarcoma cancer cells [41]. On the other hand, cyclin G is also a factor sensitizing embryonic cancer cells to drug-induced apoptosis. [PERSON] and [PERSON] in their research on the P19 embryonic cancer line showed that cyclin G influences the sensitivity of cancer cells to apoptosis after using apoptosis inducers, i.e., low serum levels, retinoic acid (RA) and TNF-\(\alpha\).