Researchers at the NIH’s National Human Genome Research Institute and Fukui Prefectural University in Obama, Japan, have created transgenic (genetically modified) versions of zebrafish by growing and genetically modifying sperm in the lab, according to a report in New Scientist. “To our knowledge, this is the first time that sperm cells have been cultured entirely in vitro and used to produce a transgenic animal,“ said NIH researcher Shawn Burgess. Zebrafish are a common aquarium fish that share many of the same genes as humans.

It’s very early days for the technology, and no one’s near to doing anything like this for mammals yet. But further development of such a procedure could eventually open the door to correcting genetic diseases before birth. The NIH news release refers to “pre-fertilization strategies for human gene therapy,“ but most people would call it “designer babies.” That’s a phenomenon that would present sticky ethical issues, as bioethicist Gregory Stock explained in a TR interview last year.