LINDA WERTHEIMER, HOST:

A new kind of genetic engineering is revolutionizing scientific research. It's letting scientists make changes in DNA in ways that were never possible before. NPR health correspondent Rob Stein has been tracking how this is raising hopes for new medical breakthroughs but also stirring deep fears. He joins us now to talk about what he's found. So, Rob, welcome to the program.

ROB STEIN, BYLINE: Nice to be here.

WERTHEIMER: First of all, let's talk about this new kind of genetic engineering. What is it exactly, and what does it let scientists do?

STEIN: So, Linda, first of all, it's got this kind of funny name. It's called CRISPR-Cas9. And what it does is it enables scientists to make changes in DNA much more easily and much faster than ever before.

WERTHEIMER: You took us over the last year to the research labs that are using this gene modification technology in controversial ways.

STEIN: So the first place I went was to a lab in California at the University of California, Davis. And scientists there are using CRISPR to do something that might sound like something out of a bad science fiction movie. They're creating embryos that are part human and part animal. And they're doing this by using CRISPR to genetically modify the embryos of farm animals, and then they're putting human stem cells into these embryos.

WERTHEIMER: Why would anyone want to do that?

STEIN: Well, there's a couple of reasons. One is if scientists can create cows, pigs and sheep that are partially human, that have partially human brains or other body parts, they could use them for medical research and maybe learn a lot about new ways to treat diseases like Alzheimer's disease or Parkinson's or diabetes. And they're also hoping that someday they might be able to grow fully functional human organs - you know, like hearts, kidneys, livers and pancreases - that could save thousands of people who are waiting for organ transplants.

WERTHEIMER: Where else did you go in your travels?

STEIN: Yeah, so I also went to Stockholm. And there, scientists at the Karolinska Institute are using CRISPR to try to edit the DNA in healthy human embryos for the first time. And they're hoping to learn more about basic human embryonic development in the hopes of learning more about what causes, say, infertility and maybe come up with new ways to help women have healthy babies.

WERTHEIMER: Well, that sounds mostly positive. Are people concerned about this one, too?

STEIN: Well, yes, they are because the concern here is that if you do it for medical research, what's to stop other scientists to try to do it for other reasons, like, for example, to try to create designer babies that are taller or smarter or better athletes? Now, we're nowhere near being able to do anything like that. But the concern is that this could open the door to someday creating genetic haves and have-nots.

WERTHEIMER: So, Rob, these CRISPR researchers are involved in projects that have tremendous promise to benefit our lives, but also raise very difficult ethical and moral questions.

STEIN: That's right. That's right. And another example I could give you is I just got back from a trip to London. And there, scientists are using CRISPR to genetically modify mosquitoes that spread a mutant gene to wipe out the mosquitoes that spread the malaria parasite. And the idea there is to try to wipe out this disease that kills hundreds of thousands of people every year, most of them children.

Now, the fear here is that they're using CRISPR to modify these insects with something known as a gene drive. And a gene drive spreads a genetic modification through an entire species really quickly. So something could go wrong. You know, it could wipe out an entire species of mosquito. And that could upset the delicate balance of an ecosystem some way that could maybe unleash some new epidemic or cause the ecosystem to collapse and cause famine.

WERTHEIMER: So how close are we to these technologies actually doing something that would touch our lives?

STEIN: So the application that probably is the closest is scientists are using CRISPR to try to genetically modify human immune system cells to attack certain cancer cells. And actually, the first patients are supposed to start receiving genetically modified human immune system cells at the University of Pennsylvania sometime next year.

WERTHEIMER: A lot of hope and a lot of room for debate. Rob Stein, NPR's health correspondent. Thank you for joining us.

STEIN: Oh, sure, great to be here. Transcript provided by NPR, Copyright NPR.