Analysis of splice acceptor site 202 in the HBV genome. A single U173G mutation affects the local RNA secondary structure. A consensus sequence of grey clade members (left panel) differs from the black collection (right panel) by an U-versus a G-nucleotide promoting exposure into a loop structure or burial into a stem structure, respectively, of a polypyrimidine tract (marked by shading) obligatory for efficient splicing at the 202 acceptor site indicated by an arrow. For the purpose of orientation, the AUG initiation codon for surface protein translation is also indicated. Values for ΔG are in kcal/mole.