embryos

Scientists are growing increasingly large and sophisticated clusters of human brain cells. Ethicists are now wondering what to do if these minibrains start thinking.

The hospital system said its investigation suggested that the problems may have been caused by human error, mechanical failure or both. "We failed our fertility clinic patients," said its CEO.

The problem hit a San Francisco clinic on March 4 — the same day that a similar cryogenic tank failure was reported in Cleveland.

Researchers disabled a gene that they think helps determine which human embryos will develop normally. The technique they used is controversial because it could be used to change babies' DNA.

The news that scientists have successfully cloned a human embryo seems almost certain to rekindle a political fight that has raged, on and off, since the creation of Dolly the sheep. It's a fight that has, over the past decade and a half, produced a lot of heat and light and not a lot of policy.