A liposome composition for delivery of a nucleic acid in vivo or ex vivo is described. The liposomes in the composition are comprised of (i) a lipid that is neutral in charge at physiologic pH and positively charged at pH values less than physiologic pH and (ii) a lipid joined to a hydrophilic polymer by a dithiobenzyl linkage. The liposomes are associated with a nucleic acid for delivery to a cell.