Cells taken from prepubescent mice have been used to successfully create sperm cells in laboratory conditions. "Takuya Sato at Yokohama City University in Japan and colleagues extracted germ cells from the testes of newborn mice that had not yet begun producing sperm. They placed the cells in agarose gel soaked in nourishing chemicals and hormones such as fetal bovine serum and testosterone. The team had first engineered the mice so that a protein only present in fully grown sperm would fluoresce green. Sure enough, around one month later, the team spotted the glowing protein in nearly half of their samples."