The present invention provides an anionic polyplex formed from a nucleic acid and an anionic polymer and further comprising a cation, which can be in the form of a nanoparticle or a microparticle and compositions comprising the anionic polyplex. The present invention further provides uses of the anionic polyplex such as delivery of the nucleic acid into cells and methods of treating a disease, disorder or condition selected from the group consisting of cancer, a metabolic, a neurodegenerative, a cardiovascular, and an infectious or inflammatory disease or disorder. The present invention also provides an anionic complex comprising a divalent cation and a nucleic acid but lacking an anionic polymer, in the form of a nanoparticle that is capable of forming a colloidal suspension.
