Authorities in the United Kingdom may soon approve clinical testing of the so-called “three-parent embryo” technique to prevent the transmission of potentially fatal genetic disease, despite ongoing concerns about its effectiveness. An advisory panel of the U.K. Human Fertilisation and Embryology Authority (HFEA), announced today that the procedure is ready for limited clinical testing, Nature reports, and HFEA is expected to make an official decision on whether to allow such trials on 15 December. The procedure, which involves transferring the nucleus from a mother’s egg cell into a donor egg containing healthy mitochondria, has already been used to create a baby, born in Mexico this year. But a study published today supports a lingering concern about the technique—that it might fail to prevent disease when a small amount of faulty mitochondrial DNA carried over in the transfer outcompetes healthy DNA in the donor cell. Researchers performed the nuclear transfer using eggs from four women with mitochondrial mutations to create 15 healthy embryos. But stem cell lines cultured from three of these embryos were dominated by the faulty maternal mitochondrial DNA—which could mean diseased tissue in a child born from such a procedure. Some scientists suggest that choosing egg donors with closely matched mitochondrial DNA could prevent the problem—a practice the new HFEA report suggests, but does not say should be mandatory.