BCL2 and cancer: In particular, we demonstrated that the 7-mer PNA having sequence ccttcct (N-to-C) complementary to the longest loop of the G-quadruplex structure formed by the 23-mer G-rich DNA sequence GGGCGCGGGAGGAAGGGGGCGGG (bcl2midG4_G, Table 1) located in the P1 promoter of the B-cell lymphoma 2 oncogene (Bcl-2) selectively binds its G4 target over the corresponding duplex tract (bcl2midG4-ds, Table 1) and downregulates the expression of the Bcl-2 gene in human A549 and MDA-MB-436 cancer cell lines [18].