Thousands of genes behave differently in the same organs of males and females, researchers reported on Friday, a finding that may help explain why men and women have different responses to drugs and diseases.

Their study of brain, liver, fat and muscle tissue from mice showed that gene expression -- the level of activity of a gene -- varied greatly according to sex.

The same is almost certainly true of humans, the team at the University of California Los Angeles reported.

"This research holds important implications for understanding disorders such as diabetes, heart disease and obesity, and identifies targets for the development of gender-specific therapies," said Jake Lusis, a professor of human genetics who worked on the study.

Writing in the August issue of Genome Research, the researchers said that even in the same organ, scores of genes varied in expression levels between the sexes.

The smallest differences were in brain tissue, they found.

"We saw striking and measurable differences in more than half of the genes' expression patterns between males and females," said Dr. Thomas Drake, a professor of pathology. "We didn't expect that. No one has previously demonstrated this genetic gender gap at such high levels."

Xia Yang, a postdoctoral fellow in cardiology who led the study, said the implications are important.

"Males and females share the same genetic code, but our findings imply that gender regulates how quickly the body can convert DNA to proteins," Yang said in a statement. "This suggests that gender influences how disease develops."

In liver tissue, the findings imply male and female livers function the same, but at different rates.

"Our findings in the liver may explain why men and women respond differently to the same drug," Lusis said.

"Studies show that aspirin is more effective at preventing heart attack in men than women. One gender may metabolize the drug faster, leaving too little of the medication in the system to produce an effect."

Yang added, "Many of the genes we identified relate to processes that influence common diseases. This is crucial, because once we understand the gender gap in these disease mechanisms, we can create new strategies for designing and testing new sex-specific drugs.