DNA tattooing was evaluated in non-human primates, which have previously shown poor DNA vaccine immunization effect, but showed remarkable enhancement of immune response by this method administering an HIV vaccine (Verstrepen et al. 2008) . In order to advance this technique to human clinical trials, a human ex vivo skin model was tested, which showed that DNA concentration was the most critical factor for effective DNA vaccination by tattooing (van den ). A human clinical trial for treating melanoma is planned (Quaak et al. 2008) . A comprehensive review on DNA tattooing can be found in one of the accompanying papers in this special volume on ID immunization (Oosterhuis et al. 2010) .