Determination of apoptotic cell percentage by a flow cytometry procedure. Sub-confluent MSCs treated with HIV-1 strains (5 ng/ml p24) and recombinant gp120 (1 μg/ml) were assayed (panel A) with propidium iodide staining after cell fixation at different times (days 1, 3, and 7). Panel B reports the apoptosis induction when hiHIV-1 strains or gp120 were used. Panel C represents the apoptotic cell percentages obtained when CD4 blockade by p5p treatment was performed. Statistical significance was determined using Student's t test with *p < 0.05.