This particular type of non-B DNA is referred to as H-DNA or intramolecular triplex DNA. Purine bases in alternating purine/pyrimidine sequences, such as GT or GC repeats, can adopt a syn conformation while the pyrimidine nucle- osides remain in an anti confirmation. Such a transition can bend the phosphate backbone into a zig-zag shape (referred to as Z-DNA) and alter the winding direction of each strand from right-handed to left-handed [9,10]. In specific sequence contexts, four guanine bases can align via Hoogsteen hydrogen bonding to form a square planar structure called a guanine tetrad [11]. Further, regions con- taining four runs of three or more guanines have the potential to form stable G-quadruplexes where three or more guanine tetrads stack with each other. Other types of non-B DNA conformations include “sticky DNA”, an intramolecular structure adopted by two triplex-like structures and A-DNA, a DNA conformation that con- tains an increase in the number of base pairs per rotation, a deeper major groove, and a shallower minor groove than B-DNA (reviewed in Ref. [2]). Some examples of non-B DNA structures are illustrated schematically in Fig. 1.