VPA is proapoptotic for ex vivo-cultured CD4⁺ T cells from HTLV-1 AC and HAM/TSP patients. CD4⁺ cells isolated from 10 AC patients (non–cross-hatched bars) and 10 HAM/TSP patients (cross-hatched bars) were cultured for 6 (D0), 24 (D1), or 48 hours (D2) with 1mM VPA (white back bars) or 5mM (gray back bars) VPA. Apoptotic cells were identified by flow cytometry based on annexin-PI labeling, and the ratios of the apoptotic rates of VPA-treated to nontreated cells were calculated. Mean ratio ± 1 SD are plotted.