Scientists have created mice with two biologically male parents for the first time — a significant milestone in reproductive biology. “This study is particularly neat because it takes advantage of errors that are known to occur during culture of XY cells, which lead to loss of the Y chromosome and subsequent gain of a second X chromosome, resulting in XX cells that are capable of generating live offspring,” said Rod Mitchell, a professor of developmental endocrinology at the MRC Centre for Reproductive Health at the University of Edinburgh in Scotland, in a statement. Only two animals of the species remain in the world, and both are female, Hildebrandt is attempting to artificially breed the animals with sperm and tissue samples taken from now deceased male counterparts