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Objectives: HIV-1 p15 Gag and its protease cleavage products, NCp7 and p6, are believed to play a major role in viral infectivity and assembly during the early and late stages of the retroviral life cycle. However, the extent to which p15 Gag is targeted by the host immune system in natural infection as well as precise cytotoxic T lymphocyte (CTL) epitopes within this protein remains to be defined., Methods: In this study, 57 HIV-1 infected individuals and 10 HIV-1 negative controls were screened for CD8 and CD4 T-cell responses using overlapping peptides spanning the entire p15 Gag protein as well as the p17 Gag and p24 Gag proteins. Peptide-specific interferon-gamma production was measured by Elispot assay and flow-based intracellular cytokine quantification, and cytotoxic activity was confirmed after isolation of peptide-specific CD8 T-cell lines., Results: CD8 T lymphocytes specific to p15 Gag were found in 46% (26/57) of HIV-1 infected individuals studied and contributed on average 17% (range, 0-100%) to the total Gag-specific T-cell responses. Responses were clustered within three immunodominant regions of p15 Gag, mapping to important functional sites. These studies also include the description of the first three optimally defined CTL epitopes within p15 Gag., Conclusions: These results indicate that p15 Gag is frequently recognized by HIV-1-specific CD8 T cells in HIV-1 infection and will be important in the comprehensive assessments of CTL responses in infected persons, as well as the design and testing of future HIV-1 vaccines and immunotherapeutic interventions.