The first mixed-embryo monkeys were born yesterday, and this is a major stepping stone in medical research. The monkeys were born in a lab in Western Oregon by the combining cells from six different embryos, BBC reports.

What is significant about this feat, is that this is the first time primates have ever been created using this method of mixing embryo cells. “Knock-out” mice would be an example of a scientific method similar to the mixed-embryo monkeys. However, this is literally a whole different animal.

To produce “knock-out” mice, scientists would take the embryonic stem cells, culture these cells in a lab, and then inject the cultured cells into a mouse embryo. With monkeys, their bodies rejected these cultured cells. But scientists discovered that they could get past this barrier by taking embryonic cells from different monkeys in order to produce the same effect.

Here is a graphical abstract of the mixed-embryo process from Science Direct:

Why should we care? Because these scientists aren’t creating mutant lab pets for pleasure. Many animals and their cells have significantly helped researchers understand and treat diseases such as diabetes, obesity, polio, and rabies.

Fun Fact: The breed of monkey that scientists used is the Rhesus monkey. This is the same monkey that has been launched into space.