From the article:

Lee and his colleagues tested 880 bioactive substances – including drugs and vitamins – approved by the U.S. Food and Drug Administration (FDA) to see if they stimulated the mouse stem cells to become heart muscle cells. The cells were genetically altered to give off a fluorescent bright green color when viewed under a microscrope if they had become heart muscle cells.

“We only got 1 out of the 880 to light up, and that was from ascorbic acid, the chemical commonly known as vitamin C,” says Lee