Humans are asymmetric animals. Early in our embryonic development, the heart turns to the left. The liver develops on the right. The left and right lungs have distinct structure.

Those ES cells carrying the inactivated gene can then be isolated and introduced (by injection) into a pre-embryonic structure called the blastocyst.

In 12 weeks, after grafting it onto five mice, the skin layer that came from human embryonic stem cells had a structure the "consistent" with human skin.