Effect of 100 nM Tat on the viability of D407 cells. The cells were incubated with 100 nM Tat and heat inactivated Tat for 24, 48, and 72 hours. Determination of the cell viability by the MTT assay indicated no significant differences between the groups (P > 0.05). Values shown represent the mean ± S.D. of four independent assays or experiments.