Charge of V3 loop sequences. a. Charges of the V3 loops of experimentally tested sequences (exp) and sequences predicted to be prototypic for their tropism (pred). The box plots show median, first and third quartile ±1.5 interquartile range. P-values displayed are based on a two-sided Wilcoxon test. b. Dot-plots showing the relationship between the net charge of the V3 loop and the probability of the corresponding strain to show the X4-capable phenotype, as calculated by geno2pheno-C_NGS-Sanger , for the set of predicted sequences. P-values based on Pearson’s product–moment correlation test: -0.17 for R5-tropic and 0.25 for X4-capable sequences.