In the study, the researchers used this technique to reprogram human skin cells. Once the skin cells had reverted to stem cells, the researchers introduced six different genetic markers that would trigger particular aspects of the cell’s development. After a month, some of the stem cells divided and matured into germ cells. However, these cells must mature more before they can become full-fledged sperm cells, as the researchers intended. Unfortunately, they found that this wasn't possible to do in the lab—the germ cells need to be implanted into a living organism to develop fully into sperm. So far, the researchers haven’t been able to do so efficiently, but they said in a press release "this is just the beginning."