The advocate-general of the European Court of Justice has recommended the prohibition on ethical grounds of patents involving human embryonic stem cells (Nature 471, 280; 2011). We write to express profound concern over this recommendation, as coordinators of multinational European stem-cell projects, working with both adult and embryonic stem cells.

Embryonic stem cells are cell lines, not embryos. They are derived using surplus in vitro fertilized eggs donated after fertility treatment and can be maintained indefinitely. As more than 100 established lines are now supplied through national and international cell banks, concern about commercialization of the human embryo is misplaced.

It is premature to suggest that human embryonic stem cells can be replaced in development of therapies. Although induced pluripotent stem cells offer additional possibilities, particularly for disease modelling, the reprogramming process is still imperfect.

Scientists working in stem-cell medicine will not be able to deliver clinical benefits without the involvement of biological industry. But innovative companies must have patent protection as an incentive to become active in Europe.

The advocate-general's opinion therefore represents a blow to years of effort to derive biomedical applications from embryonic stem cells in areas such as drug development and cell-replacement therapy. If implemented, European discoveries could be translated into applications elsewhere, at a potential cost to the European citizen. The advocate-general's opinion will now be considered by members of the court. We trust that they will deliberate on the full implications before making a legally binding ruling.