Human-animal chimeras are taking over the barnyard. A year after a research team announced they had created the first animal-human chimera by way of pig embryos containing human cells, they’ve done it again, the Guardian reports. This time it’s sheep embryos with human cells. The goal is to improve the availability and success rates of organ transplants by growing human organs inside animals. “Even today the best matched organs, except if they come from identical twins, don’t last very long because with time the immune system continuously is attacking them,” Dr. Pablo Ross says. According to the Telegraph, Ross’ team grew the embryos inside a sheep for three weeks. He says sheep have a number of advantages over pigs, including that their embryos are easier to produce via IVF.