Correlation between the exposure duration and the tipranavir mutation score. Correlations between the duration of exposure to protease inhibitors in subjects with virological failure and the tipranavir (TPV) mutation score of HIV-1 derived from these subjects, were assessed. Linear correlation was evaluated by Pearson's correlation coefficient. a. PI = Atazanavir (ATV) ± ritonavir (RTV). b. PI = Lopinavir ± ritonavir (LPV/r).