Stem cells were a type of cell found in most multi-cellular organisms, which were characterized by their ability to differentiate into a diverse range of specialized cell types.
In 2377, The Doctor used embryonic stem cells from the then-unborn Miral Paris to create an antivirus to cure nehret, a retrovirus which had infected the Klingon sect members who had traveled to the Delta Quadrant in search of the kuvah'magh. (VOY: "Prophecy")