The CpG island distribution within representative HBV sequences of HBV genotypes A-G. The open reading frames of the pre-core/core, polymerase, surface antigen, and X genes are indicated as blue arrows. The four promoters, xp, cp, sp1 and sp2, are indicated as solid yellow circles, and the main regulatory elements, enhancers I and II (Enh I and Enh II), are indicated as green boxes. The light blue areas represent the predicted methylation region of CpG islands within the HBV genome. The light blue boxes containing vertical lines represent CpG islands I to V within the HBV genome.