JUDY WOODRUFF:

When you see headlines about a new drug on the market, chances are you have rarely thought about the gender of the lab animal the drug was first tested on during trials, but, in fact, most early trials are conducted on male rats or other male animals.

Researchers say that gender difference has led to a significant impact after a drug comes to market. Last week, the National Institutes of Health announced that it is requiring scientists to test their work on both male and female animals.

For some insight into what's behind these changes and what it means, we turn to Dr. Janine Clayton. She's the director of the National Institutes of Health's Office of Research on Women's Health. That's the agency that announced the change. And Phyllis Greenberger, she's the president of the Society for Women's Health Research. She has long advocated for this change.

And we welcome you both to the program.

Dr. Clayton, let me start with you.

Why does the sex, the gender of the animal or the cell where the test is being conducted make a difference?

DR. JANINE CLAYTON, National Institutes of Health: The sex of the cell makes a huge difference, because the pre-clinical studies, where we're testing drugs or therapies, are those studies that build the evidence base and inform the clinical studies.

So if you are going to be studying a disease that affects both men and women, it's really important to think about male and female cells and males and females in the animal model work when you are doing that pre-clinical research.