First, a little lesson to bring you up to speed: chimeras are organisms comprised of cells from two or more species. Essentially, species are combined—two sets of DNA, two separate organisms—into one. The word is plucked from Greek mythology, referencing a creature that is part dragon, part goat, and part lion.

Attempts to create chimeras have been around since the 1980s. Now, a new type of chimera created in China, monkey embryos containing human cells, is bringing up a new set of ethical concerns.

Led by Professor Juan Carlos Izpisúa Belmonte, the team is comprised of scientists from the Salk Institute in the United States and Murcia Catholic University (UCAM) from Spain. The chimera-creation process first involves genetically tweaking a monkey embryo’s cells to stop organ development. Human stem cells are then infused into that monkey embryo.

Alejandro De Los Angeles, a researcher from Yale University’s Department of Psychiatry, told The Guardian that monkey-human chimeras are likely to be used to discover the best proportion of human to animal cells. Researchers can then translate this “with the hope of making organs for transplantation,” De Los Angeles said.

Previously, Belmonte and several other scientists have managed to do the same with pig and sheep embryos, to a lesser degree of success. The pig embryo only contained one human cell for every 100,000 pig cells, and the human cells didn’t take hold.

Some scientists, such as Professor Robin Lovell-Badge of the Francis Crick Institute in London, believe that this project comes with inherent ethical concerns.

“How do you restrict the contribution of the human cells just to the organ that you want to make?” he said. “If you allow these animals to go all the way through and be born, if you have a big contribution to the central nervous system from the human cells, then that obviously becomes a concern.”

In this case, De Los Angeles, who is not involved with the project, also stated that the human-monkey chimeras were only allowed to develop for mere weeks before any organs fully formed. If they had developed completely, there would be more cause for controversy.

The team of Spanish and American researchers chose China because they could avoid legal issues which are more likely in Spain. In the latter, research involving chimera-creation and animal modification is only permitted when it involves deadly diseases.

“The ultimate goal would be to create a human organ that could be transplanted but the path is almost more interesting for today’s scientists,” said Estrella Núñez, a member of the team and the Vice-Chancellor of Research at UCAM. “What we want is to make progress for the sake of people who have a disease.”