The first successful human–animal hybrid has been created: a mixture of human and pig. Jun Wu of the Salk Institute in the US and colleagues built on previous work making chimeras from mice and rats, and managed to put human cells into developing pig embryos. The embryos were then inserted into adult pigs, which carried them for three to four weeks, after which they were removed and studied. Although considered controversial, the work demonstrates that human cells can be made to survive and grow inside of a host animal, bolstering hopes for better ways to make organs without the need for human donors.