HIV-1 has demonstrated the ability to cause adverse neurological complications. The introduction of highly active antiretroviral therapy (HAART) has resulted in improved survival, reduced viral loads, increased CD4 T- cell counts, reduced opportunistic infections, and has resulted in a decrease in the most severe form of HIV- associated neurocognitive disorders (HAND), HIV-associated dementia (HAD). However, the overall prevalence of all forms of HAND has increased, potentially because HIV-1-infected individuals are living longer with persistent infection. In addition to activating an antiviral immne response within the CNS, the production of HIV-1 proteins leads to neurotoxicity through direct and indirect mechanisms. Of particular interest is the HIV-1 transactivator protein Tat, which in addition to driving viral transcription also functions as an extracellular neurotoxic protein, activator of astrocytes and microglial cells, and impacts the structure and function of the blood-brain barrier (BBB). Tat is released by infected monocytes, microglial cells, and astrocytes in addition to infected lymphocytes. Tat can be continually produced by HIV-1-infected cells located in the CNS despite the widespread use of HAART, demonstrating that neurologic vulnerability to this protein persists in the HAART era. In addition, mRNA levels for Tat are elevated in brain extracts from individuals with HAD. HIV-1 also displays extensive sequence variation with the quasispecies shaped by many host-specific and comorbidity pressures, while simultaneously maintaining functions that are critical to replication and infectivity as well as pathogenesis. Studies with respect to subtype B Tat genetic variation and HAND have demonstrated (i) Tat sequences from patients in the absence or presence of HAD showed clustering of sequences with respect to neurological impairment as well as tissue of origin, (ii) nonsynonymous versus synonymous mutation rates among brain-derived Tat sequences from patients with HAND were significantly greater than those isolated from patients without neurological disease, and (iii) variation at positions 74 and 100 were correlated to Tat sequences isolated from brain-derived sequences. Importantly, HIV-1 Tat derived from HAD patients has been associated with greater neuronal death. Together, these reports suggest that genetic diversity of HIV-1 Tat likely contributes to the establishment and severity of HAND. Given this, the hypothesis of this application is that patient-derived genetic variants of the HIV- protein Tat are specifically linked with neurologic impairment. To explore this hypothesis, the Specific Aims are to: (1) identify and characterize polymorphisms in HIV-1 Tat from patients with defined degrees of neurocognitive impairment; (2) examine the structural impact of HIV-1 Tat polymorphisms in protein-protein molecular models; and (3) examine the functional impact of the Tat polymorphisms on the BBB and cells of the CNS. These studies will contribute to defining how HIV-1 Tat polymorphisms affect the function of the CNS and the development of HAND and provide information useful in development of diagnostic assays of HAND and potential preventative and treatment strategies.