One way to alleviate the current shortage in donor organs is to generate human organs within animals by a process called blastocyst complementation. However, organs are complex tissues that include blood vessels. Contamination of animal-derived blood vessels in the human organ could result in rejection following transplantation. Here, we have studied a new way to generate rejection-free transplantable organs by blocking blood vessel formation within the host, by genetically deleting a gene called Flk-1.