Sir

In their Commentary “The future of gene therapy” (Nature 427, 779–781; 2004), M. Cavazzana-Calvo and colleagues suggest that we proposed, in a recent Perspectives article (Science 302, 400–401; 2003), screening genetically modified stem cells before they are infused into patients, to avoid potential serious side effects.

We are very much aware that such an approach is not applicable with current methods. Our comment represented our vision of where the field needs to move in the future. To thrive, the field needs both continuous development of realistic clinical trials — as outlined by Cavazzana-Calvo and colleagues — and conceptual discussions about new strategies. Regulatory decisions need to follow the former, scientific the latter.

As outlined in the Perspectives article, advanced cell culture, gene delivery and molecular screening technologies are likely to bring significant progress in the foreseeable future.