DNA vaccines induce immune responses against antigens synthesized in vivo after direct introduction of the DNA's encoding sequences. This unique approach to immunization may overcome deficits of traditional antigen-based approaches and provide safe and effective prophylactic and therapeutic vaccines. DNA vaccines are also useful as a research tool, such as for production of monoclonal antibodies. Efforts are now focusing on understanding the mechanism of antigen presentation and the adjuvant effect of immunostimulatory CpG motifs in the vectors to aid optimization of DNA vaccines.