For each group, enrollment started with five participants per group. All participants were evaluated for PK and safety criteria, adjusting the dose of ATV until one was found that passes both sets of criteria. Then five additional participants were enrolled, with enrollment continuing for each group once all participants within that group meet the PK criteria. For groups receiving RTV (Groups 5 to 8), additional criteria must be met for each dose of ATV studied. In addition to the PK and safety evaluations, 24-hour post-dose concentrations (Cmin) were monitored in the first 10 participants enrolled for a dose of ATV before more participants were enrolled and studied at that same dose. Note that in Protocol Version 5.0, South African (S.A.) sites were allowed to enroll patients in study groups 3,4,5,6,7,8. As a result, the study design has been modified to further stratify study groups 3, 4, 5, 6, 7, 8 (at the final recommended dose), by country, i.e., U.S.A. versus S.A., such that 10 evaluable study subjects will be accrued in parallel to each study group-country cohort.