Figure 3. Importance of positions of normalized V3 sequence in random forest classification with Kyte-Doolittle descriptor [20].

The higher the peak at the respective position, the more important this position for correct classification of sequences with respect to co-receptor tropism. The most important region is around normalized sequence position 12, in agreement with the 11/25 rule. The second most important region around position 8 could be involved in binding of sulfated tyrosine on CCR5 [23]. Along the top axis, reference sequence HXB2 before normalization is given for orientation.