MYC and cancer: Remarkably, the promoter region of c-myc—an oncogene over-expressed in the majority of solid tumors and closely associated with cancer cell apoptosis, proliferation, invasion, cell-cycle arrest, and metastasis—can form a parallel G4 structure via Hoogsteen hydrogen bonds under specific conditions, and has been proposed as an effective target for antitumor drugs [15,16,17,18,19,20,21,22,23].