VPA activates the expression of the virus core-protein p19 in the culture supernatants of CD4⁺ T cells from HTLV-1–infected individuals. Culture supernatants were collected and expression of virus core-protein p19 was quantified by ELISA. Absolute p19 concentrations (in picograms per milliliter) were determined by normalization of absorbance values to a standard curve. p19-protein levels were corrected by considering the proportion of nonapoptotic cells, determined from annexin-PI flow cytometry analysis of each sample. Mean ± 1 SD concentration at different incubation times is represented for the 20 HTLV-1–infected samples cultured without (black bars) or with 5mM VPA (cross-hatched bars).