It turns out that the human genome contains about 100,000 fragments of endogenous retroviruses, making up about eight percent of all our DNA.
 

 

But over the past four years, Sasai has used mouse embryonic stem cells to grow well-organized, three-dimensional cerebral-cortex1, pituitary-gland2 and optic-cup3 tissue. His latest result marks the first time that anyone has managed a similar feat using human cells.