At Harvard, they bred genetically manipulated mice that lacked an enzyme called telomerase that stops telomeres getting shorter.

The researchers say they have bred mice missing a gene that bears instructions for making an important brain protein.

When researchers bred mice without the huntingtin gene, they died before birth, Dr. Wexler said.

They can be handled easily, readily accept captive bred mice and rats for food, but are shy captives and do best when left alone.

Dr. Jeremy Nathans, a professor at the medical school, with Nini Guo and Charles Hawkins, bred mice that lacked the gene, called Frizzled6.

They bred mice in which they could selectively stop these neurons by injecting a virus into specific areas of the brain.

The English love hedgehogs and butterflies and they breed champion mice for exhibition.

He bred mice that had been exposed to the hormone prenatally, when their mothers were injected, with males never exposed to it.

By breeding these "knock-in" transgenic mice, they generated heterozygous females with both an M cone and an L cone.

Through an intricate series of manipulations, they breed mice that lack copies of the genes under study and then see how the rodents fare.