A fragment of 208 bp, containing the whole short Pol b 3 0 UTR sequence, generated as described in Structural analysis of RNA, was inserted downstream of Firefly luciferase in the pCMLuc vector (pCM2 derivative, a gift from Dr D.Weil, Institute Andre Lwoff, Villejuif, France) into the EcoRI site present in the polylinker, generating pCMLucH. The same fragment containing a mutation of the hairpin-forming region was cloned into the EcoRI site of pCMLuc, generating pCMLucHmut. The rat hepatoma FTO-2B cell line was grown on DMEM (Invitrogen) supplemented with 10% FBS (Invitrogen). Transfection of FTO-2B was performed using Lipofectamine 2000 (Invitrogen), according to the manufacturer's guidelines. Two hundred nanogram of the appropriate plasmid was used for transfection, performed in 50 ml of medium in a 96-well plate. The activity of Firefly luciferase in lysates prepared from cells transfected with pCMLuc, pCMLucH and pCMLucHmut was measured using a luciferase reporter assay system (Dual-Glo, Promega). Assays were performed according to the manufacturer's instructions. Light emission was measured with LumiCount microplate luminometer (Packard). Transfection efficiencies were normalized by co-transfection with phRL-CMV vector (Promega) containing the Renilla luciferase.