Knockdown of the HTLV-1 oncoprotein Tax in HTLV-1-transformed cells abrogates proliferation. ATL-derived (HuT-102, HPB-ATL2, and MT1) or HTLV-1-transformed cell lines (C91PL and MT2) were transduced using GFP-lentiviral vectors encoding SCR shRNA or 2 (HPB-ATL2, MT1, C91PL, and MT2) or 3 (HuT-102) different shRNAs against Tax (sh1-Tax, sh2-Tax, and sh3-Tax). Growth of transduced GFP⁺ or untransduced GFP⁻ sorted cells, as well as of CEM cells (HTLV-1 negative T-cell line), was assessed by cell count using trypan blue staining up to 7 days after sorting. For simplicity, the results obtained from independent experiments using different individual shRNAs were pooled together.