It has been widely accepted that DNA can adopt other biologically relevant structures beside the Watson-Crick double helix. One recent important example is the guanine-quadruplex (G-quadruplex) structure formed by guanine tracts found in the MYC (or c-myc) promoter region, which regulates the transcription of the MYC oncogene. Stabilization of this G-quadruplex by ligands, such as the cationic porphyrin TMPyP4, decreases the transcriptional level of MYC. Here, we report the first structure of a DNA fragment containing five guanine tracts from this region. An unusual G-quadruplex fold, which was derived from NMR restraints using unambiguous model-independent resonance assignment approaches, involves a core of three stacked guanine tetrads formed by four parallel guanine tracts with all anti guanines and a snapback 3'-end syn guanine. We have determined the structure of the complex formed between this G-quadruplex and TMPyP4. This structural information, combined with details of small-molecule interaction, provides a platform for the design of anticancer drugs targeting multi-guanine-tract sequences that are found in the MYC and other oncogenic promoters, as well as in telomeres.