HTLV-1 infected FS (A), SP (B) and HUT102 (C) cells were treated with PMA for 30 min, washed, then treated with Act D, and RNA decay assays were performed. Tax/rex mRNAs were measured with splice site-specific primers (closed circle: untreated, open circle: PMA treated). Similar to results observed in FS cells, PMA also increased the tax/rex RNA stability in SP cells. In contrast, in HUT102 cells, which constitutively express high levels of Tax protein, PMA fails to change the RNA degradation rate.