A very short form of RNA in the nucleus of cells. Micro RNA molecules are typically only 20-25 base units long. They are transcribed from DNA, but are not involved directly in protein synthesis like other forms of RNA. They perform important functions similar to enzymes in regulating chemical reactions in our cells, especially in the embryonic stage at the beginning of life. It is thought that 1/3 or more of human genes are controlled in some way by micro RNA molecules. At least 200-255 human DNA genes code for the production of micro RNA molecules.