What if we could improve the human gene pool?    Create children that are free of the genes that cause inherited diseases, like Huntington’s or certain kinds of cancer, by modifying the DNA in eggs, sperm, or embryos?   Well, it’s possible.   The media have been talking about it lately in terms of “genetically modified humans” or “designer babies.”   But to scientists, it’s known as germ-line engineering -- referring to the germ cells, or sperm and egg cells that come together to create an embryo.    Putting an end to heritable disease sounds like a crowd-pleasing idea, right?   But germ-line engineering is pretty controversial. In fact, about 40 countries either discourage or outright ban the practice.    And yet, researchers continue to inch closer to performing germ-line engineering experiments on humans.   This has prompted some geneticists, in the new issue of the journal Nature to call for a moratorium on germ-line engineering.    And in this week’s journal Science, other researchers voiced their concerns, too, saying that more people -- including the general public -- have to get involved in the discussion before we start editing people’s sperm and egg cells.   So how does germ-line engineering work? And what should I think about it?   Okay, I’m not going to tell you what to think, but I will tell you what you need to know.    [Intro]   Since scientists first began sequencing the human genome in 1990, they have identified thousands of genes that increase our risk for certain diseases.   Take ovarian and breast cancer, which can be linked to the mutation of a gene called BRCA1.    This got some researchers to thinking: What if we could swap out that mutated gene, and put in a new one?    And what if you replaced that mutated gene before an embryo even developed -- like, in an egg or sperm cell?   Then you could use that cell to create an embryo with a healthy BRCA1 gene, drastically reducing the risk of those cancers... and not just for one embryo, but for its offspring, too.   In 2013, scientists found a way to do this, using a system called CRISPR.   The technology is actually based on the way bacteria defend themselves against viruses.   When a bacterium detects that a virus has infiltrated it, it releases what’s known as guide RNA, a strand of genetic information that contains a sequence that matches part of the virus’ DNA.    And attached to it is an enzyme, called cas9.    The guide RNA acts like a homing device, leading the enzyme to the matching segment of viral DNA.   When the two strands meet, the enzyme goes into action, acting like a pair of scissors to snip out that section of viral DNA, rendering it useless.    Scientists discovered that they could imitate this process, modifying guide RNA to find any sequence of DNA in any organism, including humans.    So, theoretically, they could inject a segment of DNA that represents a healthy, unmutated gene and use that to replace a mutated one in an egg or sperm cell.    The genome of that germ cell would be permanently edited, and so would all of the cells in the organism that it went on to form.    This method has been tested on rats and monkeys, and lately talk has turned to seeing if it is possible with human germ cells.    Researchers at Harvard, for instance, say they’ve considered obtaining an egg from a patient with ovarian cancer, and using CRISPR to replace the BRCA1 gene in the egg with a gene without the mutation.    That’s when five prominent geneticists showed up in the journal Nature, calling for a moratorium.    So. What’s the problem?    Well, for one thing, they say, germ-line engineering is unpredictable.   In an experiment on monkeys done at MIT, it only succeeded about 20 to 40 percent of the time.    And when it doesn’t work, it can accidentally delete parts of the genome that weren’t targeted, potentially creating new mutations.    Plus, they point out that CRISPR isn’t the only way to make sure that a child doesn’t inherit a mutation.   With methods like in vitro fertilization -- where the egg is fertilized outside the womb -- embryos can be tested for mutations before they’re implanted into the mother, so no offspring can get the bad gene.    Now, to be clear, the critics of germ-line engineering don’t have any problem with tinkering with people’s genes in general.    In fact, the authors of the op-ed in Nature are some of the leading figures of somatic cell engineering.   Somatic cells are the non-reproductive cells in our bodies, and somatic cell engineering aims to eliminate mutations in enough of those cells to reverse the course of some diseases.    This technique can use CRISPR, too, but its effects would be non-heritable. So it wouldn’t transfer to the offspring.   The authors are also worried that public concerns about genetically engineering embryos could have a negative effect on their less-controversial, but similar-sounding, research.   Until scientists, the public, and the legal system can all figure out how we’re going to deal with germ-line engineering, they want researchers to pause developing the technology.   So, those are the facts.   Science will never be a stranger to controversy, so neither should your brain.   What do you think? Let us know on Facebook or twitter or in the comments below. And if you want to keep understanding the science behind the headlines, head over to Patreon.com/scishow to see how you can become a SciShow Patron.