Several groups have demonstrated that G-rich oligonucleotides forming G-quartet structures display activity as potential drugs, such as potent HIV inhibitors. The delivery of G-quartet oligonucleotides to their intracellular targets is a key obstacle to overcome for their clinical success. Here we have developed a novel system to deliver G-rich oligonucleotides into the cell nucleus, e.g., the site of HIV integration. On the basis of the property of potassium-induced formation of G-quartet structure, we explored the difference of K(+) concentrations inside (140 mM) and outside (4 mM) cells to induce the G-rich oligonucleotides to form different structures inside and outside cells. The key steps of this delivery system include the following: (i) First, the G-quartet structure is denatured to form a lipid-DNA complex, so that the molecules can be well delivered into cells. (ii) Then the delivered molecules are induced to form G-quartet structures by potassium inside cells since the G-quartet structure is the primary requirement for inhibition of HIV-1 HIV integrase (IN) activity. The molecules of a novel G-quartet HIV inhibitor, T40214, with the sequence of (GGGC)(4) were successfully delivered into the nuclei of target cells, which significantly decreased HIV-1 replication and increased the probability to target HIV-1 IN in infected cells.