Scientists at the University at Buffalo and the Roswell Park Cancer Institute have bred a new form of human-mouse chimera with the highest incidence of human cells ever recorded. Chimeras are organisms made up of a mixture of genetically different tissues—in this case, mouse cells and human stem cells. The team published its work in the journal Science Advances.

Two weeks after the researchers injected human stem cells into the developing mouse embryos, one of the newborn mice exhibited 4 percent human cells—a major advance, considering human and animal cells don't typically jive well. While they're still mostly just mice—and only a tad bit human—the breakthrough marks a step toward more advanced genetically modified embryos in the future.