Figure 1: Algorithms for determining HBV subtype from the primary structure of the HBV S gene. A. The decision tree for the ad subtypes. B. The original decision tree for determining the ay subtypes. C. A newly developed decision tree for determining the ay subtypes. Each box in each decision tree contains a number representing an amino acid position in the S gene. The amino acids listed below each number represent the permitted wild type residues at that position. The downward pointing triangles denote paths through each decision tree. The fi nal designation at the end of each possible path is the subtype for that path.