An advisory committee of the Food and Drug Administration is set to begin two days of meetings tomorrow to consider radical biological procedures that, if successful, would produce genetically modified human beings. This is a dangerous step. These techniques would change every cell in the bodies of children born as a result of their use, and these alterations would be passed down to future generations. The F.D.A. calls them mitochondrial manipulation technologies. The procedures involve removing the nuclear material either from the egg or embryo of a woman with inheritable mitochondrial disease and inserting it into a healthy egg or embryo of a donor whose own nuclear material has been discarded. Any offspring would carry genetic material from three people - the nuclear DNA of the mother and father, and the mitochondrial DNA of the donor. Developers of these modification techniques say they are a way for women with mitochondrial disease to give birth to healthy children to whom they are related genetically. Some are also promoting their use for age-related infertility. These procedures are deeply problematic in terms of their medical risks and societal implications. Will the child be born healthy, or will the cellular disruptions created by this eggs-as-Lego-pieces approach lead to problems later on? What about subsequent generations? And how far will we go in our efforts to engineer humans? Unfortunately, there are now worrisome signs that opposition to inheritable genetic modifications, written into law by dozens of countries, according to our count, may be weakening. British regulators are also considering mitochondrial manipulations, and proponents there, like their counterparts in the United States, want to move quickly to clinical trials.