WASHINGTON: What makes a man different from a mouse? Genetically, it is pretty hard to tell, researchers said recently.

An initial comparison of one mouse chromosome to a human chromosome shows the genes they carry are highly similar, a team at genome company Celera Genomics reports in Friday’s issue of the journal “Science.”

Scientists hope that by comparing human DNA sequences to those of other animals, they can tease out what it is that makes us unique.

And because scientists have experimented on billions of lab mice and know a great deal about their genetics, they hope the field, called comparative genomics, can help them better understand human biology.

Richard Mural and a team of colleagues at Celera compared chromosome 16 in the mouse to human chromosome 21, which it closely resembles. Mural said both are fairly small and well-understood, which is why his team compared them.

“To me, the thing that I found the most interesting is just how similar the mouse and the human are in respect to genes, and gene content and DNA sequences,” Neal Copeland, an expert in genetics and genomics at the National Cancer Institute, said.

The Celera researchers found mice have about 10 per cent less DNA than humans, mostly because the human genome has a great deal of repetitive sequences, once called “junk DNA.”

Celera is sequencing the entire collection of mouse genes. It is also finishing the human sequence, which it assembled in 2000. A publicly funded effort to sequence and analyze the entire mouse genome in also underway.

GENOME MYSTERY: One of the mysteries raised by sequencing the human genome is how few genes it takes to make a person. Scientists once thought humans had about 100,000 different genes, but Celera and the publicly funded Human Genome Project came up with about one-third that number.