A four-week-old pig embryo injected with human pluripotent stem cells. (credit: Salk Institute, JUAN CARLOS IZPISUA BELMONTE)

Scientists have successfully created pig embryos that contain a small fraction of human cells, according to a study published Thursday in Cell.

The study, led by researchers at the Salk Institute in California, represents a significant step toward human-animal hybrids that could one day grow whole new human organs for transplant. Right now, the pig hybrids only contain about one human cell to 100,000 pig cells and were very tricky to make. And due to ethical guidelines, the researchers only let them develop for about a month. But researchers are optimistic that they’ll be able to tackle the technical—and ethical—challenges moving forward.

They certainly have a lot of hints that they’ll be successful.