When my dad had a heart valve replaced, the doctors gave him a choice: a mechanical valve, or one from a pig? In his case, the surgeon recommended metal over meat. Now, thanks to CRISPR engineering scientists have successfully created piglets free of certain viruses, and humans are now one step closer to receiving not just pig valves, but entire porcine organ transplants.

Why it matters: "In the U.S. alone, more than 116,000 people are waiting to receive a lifesaving organ transplant, while only 17,157 transplants have been performed this year," writes Emily Mullin at MIT Technology Review. For years, doctors and scientists have hoped organs grown in other animals could fill that gap, but so far they've been unsuccessful.

Why they did it: Pig organs are good candidates for human transplants because they're a similar size to humans and pigs are easy to farm, but until recently full organ transplants have been too dangerous: pigs have a virus, porcine endogenous retrovirus (PERV), incorporated into their genome. Since pigs and humans can easily share diseases (think: the flu), PERV has been a barrier to transplants of more complex pig-grown body parts.

But there are still ethical barriers. Gina Kolata at the New York Times writes "the prospect also raises thorny questions about animal exploitation and welfare. Already an estimated 100 million pigs are killed in the United States each year for food." Religious groups, medical advocacy groups and animal rights groups have weighed in on various sides of the debate.

Right now, transplants come with a price: rejection. Sarah Zhang at The Atlantic notes that pig organs are currently incompatible with human ones. They require anti-rejection drugs. But eGenesis, the company that created the pigs, says they are actively identifying genes they could change to make the organs more compatible. Ultimately, they want a human to receive a pig transplant without any anti-rejection drugs.