Box 1. Spanners in the works

Cloning is notoriously inefficient — it can take hundreds of eggs to produce a single embryo. Researchers trying to work out why can see one possible culprit: the tiny bodies that generate a cell's energy.

These powerhouses, called mitochondria and pictured right, have their own small genomes. But they also need to interact with the products of genes in the nucleus. A mismatch between mitochondrion and nucleus, or even between resident mitochondria and those introduced during the cloning process, could make the cell fail.

"Mitochondria are being totally overlooked — with devastating consequences," says Doug Wallace of the University of California, Irvine.

Fusing cells from two individuals might cause problems, says Wallace, but the biggest problems will arise if researchers try using animal eggs to reprogramme human nuclei. "From our experience, combining the mitochondrial DNA from even a species as closely related as chimpanzees result in incompatibilities."

A recent study suggests that cellular function may not be affected in cells derived from mice clones. But Wallace argues that such cells might be compromised when they move from the carefully controlled lab environment to repairing dilapidated organs inside a body.