FANCJ helicase is a human protein, which participates in DNA repair by homologous recombination. Mutations of FANCJ or changes in its expression levels are linked to Fanconi anemia, ovarian and breast cancer. Fanconi anemia is accompanied by a higher susceptibility to cancer and genomic instability. FANCJ helicase is encoded by the BRIP1 gene, which contains a guanine rich sequence capable of forming G-quadruplexes in its promotor region, near the transcription start site. In this master’s thesis, we have confirmed that a 32 nucleotides long guanine rich sequences, which originates from the FANCJ helicase promoter region, can form G-quadruplexes in solution. With nucleotide substitutions, we have forced the oligonucleotide to adopt a single G-quadruplex structure. Using one- and two-dimensional NMR spectroscopy methods, we have determined that an oligonucleotide with two substituted nucleotides forms a G-quadruplex that adopts a hybrid type 2 topology. Using melting curve analysis, we have found out that its thermal stability is similar to the thermal stability of the parent G-quadruplex. We have studied the importance of the length of a propeller loop in the hybrid type 2 topology G-quadruplex and shown that shortening of the propeller loop leads to a change of structure from a three-quartet to a two-quartet G-quadruplex. We have observed that guanine-rich sequences are abundant in promoter regions of other human helicases, which unfold G-quadruplexes. We have also proposed a possible biological G-quadruplex – helicase system, which could work as a feedback loop.