An extra step could eliminate many objections to the controversial reproductive cloning technique.

Cloned human embryos would be used to derive healthy sperm and eggs that could subsequently be used for in vitro fertilization rather than implanting the embryo directly into the mother’s uterus.

Embryonic stem cells would be removed and nurtured in the lab. Researchers would then treat the cells with molecules that encourage them to develop into either sperm or eggs. The resulting cells could be fused with the sperm or egg of the other partner by in vitro fertilization, and the embryo could then be implanted.