Building on the high-tech science that lets couples choose their babies' sex or screen for genetic disorders, researchers are reporting the birth of five children from embryos created in a lab so they could serve as cell donors to seriously ill siblings. Scientists at Chicago's Reproductive Genetics Institute say this is the first time embryos have been screened to ensure that the baby could donate life-saving stem cells to a needy brother or sister. Yury Verlinsky, the institute's director, said all babies were healthy and that the single transplant that has taken place was a success. The new research, to be reported in Wednesday's Journal of the American Medical Association, will add fuel to the debate over the ethics of conceiving a child for use as a donor and creating so-called