A formulation for use in a method of treating the human or animal body, which method comprises the delivery of an active agent into a cell, and wherein the formulation comprises a composition of at least one lipid moiety covalently linked to one or more transfection enhancer elements (TEE's), wherein the at least one lipid moiety comprises a sterol and wherein the one or more TEE's are defined according to the formula: hydrophobic moiety − pH sensitive hydrophilic moiety and wherein the pH sensitive hydrophilic moiety of each of the one or more TEE's is independently a weak acid having a pKa of between 2 and 6 and is located distally from the link between the lipid moiety and the TEE, and wherein the hydrophobic moiety of each of the one or more TEE's is independently a straight alkyl carbon chain comprising 12 to 20 carbon atoms; wherein the composition comprises the general formula: wherein Xa is bound at the 3' position of the sterol and is -OPO 3 - -; -OPO 2 - CH 2 -; pyrophosphate-; -OSO 3 -; -OSO 2 -; -O-; -NH-; -S-; -N C -; -C -, -C N -; -OC N -; -N C O-; -OC -; -C O-; -N=CH-; -CH=N-; -CH 2 -; -S-S-; -N(R 13)(R 14)-; wherein R 13 and R 14 are independently H or lower alkyl and wherein one of R 13 or R 14 may be absent; Ya is absent or a non-branched, branched or cyclic substituted or unsubstituted alkyl, alkenyl, alkylene, alkynyl, or an aryl group with 1 to 8 carbon atoms, wherein Ya is substituted with one or more groups selected from -OH; -NH 2; -N(R 13)(R 14); -COOH; a sugar; an amino acid; a di- or tripeptide; an α-hydroxyacid, a β-hydroxyacid or a dihydroxyacid; wherein R 13 and R 14 are as defined above; Za is absent or selected from -O-; -S-; -N C -; -C N -; -C O-; -OC N -; -C -; -C N -; -N C O-; -NH-; -N(R 13)(R 14)-; -CH=N-; -OC -; -N=CH-; -S-S-; -NH 2; -(NH 3) +; -(N(R 13)(R 14)(R 15)) +; -C OH; -OH; -C H; -C OR 16; -C NH 2; wherein R 15 is defined as R 13 and R 14 above and wherein R 16 is lower alkyl; wherein the sterol is selected from cholesterol, sitosterol, campesterol, desmosterol, fucosterol, 22-ketosterol, 20-hydroxysterol, stigmasterol, 22-hydroxycholesterol, 25-hydroxycholesterol, lanosterol, 7-dehydrocholesterol, dihydrocholesterol, 19-hydroxycholesterol, 5α-cholest-7-en3β-ol, 7-hydroxycholesterol, epicholesterol, ergosterol, and dehydroergosterol; wherein "lower" means 1-6 atoms; and wherein the dotted lines represent possible graft positions of the one or more TEE's. wherein R 13 and R 14 are independently H or lower alkyl and wherein one of R 13 or R 14 may be absent; Ya is absent or a non-branched, branched or cyclic substituted or unsubstituted alkyl, alkenyl, alkylene, alkynyl, or an aryl group with 1 to 8 carbon atoms, wherein Ya is substituted with one or more groups selected from -OH; -NH 2; -N(R 13)(R 14); -COOH; a sugar; an amino acid; a di- or tripeptide; an α-hydroxyacid, a β-hydroxyacid or a dihydroxyacid; wherein R 13 and R 14 are as defined above; Za is absent or selected from -O-; -S-; -N C -; -C N -; -C O-; -OC N -; -C -; -C N -; -N C O-; -NH-; -N(R 13)(R 14)-; -CH=N-; -OC -; -N=CH-; -S-S-; -NH 2; -(NH 3) +; -(N(R 13)(R 14)(R 15)) +; -C OH; -OH; -C H; -C OR 16; -C NH 2; wherein R 15 is defined as R 13 and R 14 above and wherein R 16 is lower alkyl; wherein the sterol is selected from cholesterol, sitosterol, campesterol, desmosterol, fucosterol, 22-ketosterol, 20-hydroxysterol, stigmasterol, 22-hydroxycholesterol, 25-hydroxycholesterol, lanosterol, 7-dehydrocholesterol, dihydrocholesterol, 19-hydroxycholesterol, 5α-cholest-7-en3β-ol, 7-hydroxycholesterol, epicholesterol, ergosterol, and dehydroergosterol; wherein "lower" means 1-6 atoms; and wherein the dotted lines represent possible graft positions of the one or more TEE's.