WASHINGTON — Scientists have engineered mice that have up to 4 percent of their body made of human cells, or the highest amount achieved in human-mice chimeras, according to a study in Science Advances.

To create the chimeras, scientists took human stem cells and chemically suppressed a protein called mTOR for three hours. This forces the cells to revert back to the 'naive' state, which enables them to grow inside mice embryos.

As the embryos matured, researchers found the tissues that would become the heart, liver and bone marrow to contain human cells, which were also found in the blood. Science News reports the technology may help scientists to use chimeras as a source for human organs.

Scientists did not detect any trace of human material in the cells that would develop into mice sperm or egg. This eliminates ethical concerns of chimeras capable of reproduction.