The cause of bone disease is most often an imbalance between bone resorption and bone formation. The most common metabolic bone disease is osteoporosis, which can occur because of disorders in the two main regulatory mechanisms. The first one is the RANKL/RANK/OPG system, which is the most important in the control of osteoclastogenesis; and the second one is the Wnt signalling pathway, which plays a role in osteoblastogenesis. In this master thesis, we focused on the RANKL/RANK/OPG system. When this system is not working properly, it can result in increased expression of the RANKL protein, which binds to the membrane of osteoclasts, and can increase their number and activity. The gene encoding the RANKL protein is called TNFSF11. The promoter region of the gene TNFSF11 contains regulatory elements and a structure called G-quadruplex. The regulatory elements enable the binding of various transcription factors that can accelerate or inhibit the expression of the TNFSF11 gene. In this thesis, we studied the influence of changes in the G-quadruplex sequence on the activity of the TNFSF11 gene promoter. Using the plasmid pGL3 vector, which contained part of the TNFSF11 gene promoter region, we inserted mutations on different parts of the G-quadruplex (RANG2713T, RANG2719T and RANG27T). With transfection, we then introduced plasmids into HeLa cells (cervical cancer cells), and we measured the effect on the promoter activation using a double luciferase assay. In comparison to the original plasmid, all the prepared plasmids with mutations showed a decrease in the promoter activity (In the RANG2713T and RANG2719T mutations, the activity decreased by a factor of 0,2 and in RANG27T by a factor of 0,4). Decreased promoter activity means lower RANKL protein expression and consequently bone resorption. We also studied the influence of two genetic alterations in the TNFSF11 gene – rs201748497 and rs200072190, replacing one base pair in both, which also led to a decrease in this gene’s promoter activity (in comparison to the original plasmid, the promoter activity in rs201748497 decreased by a factor 0,9 and in rs200072190 by a factor 0,7). According to the obtained results, we can conclude that every alteration we introduced affects the change of the TNFSF11 gene promoter activity, consequently the protein RANKL expression, and thus the occurrence of osteoporosis. This provided new information on the impact of individual changes in the TNFSF11 gene; which could help to develop more effective and selective agents that would bind either to the G-quadruplex or any other system involved in the bone repair process (RANK/RANKL/OPG).