Mediator of RNA polymerase II transcription, subunit 12 homolog (S. cerevisiae), also known as MED12, is a human gene found on the X chromosome. Mutations in MED12 are responsible for at least two different forms of X-linked dominant mental retardation, Lujan-Fryns syndrome and FG syndrome, as well as instances of prostate cancer. Mutations in MED12 are associated with uterine leiomyomas and breast fibroepithelial tumors (e.g. fibroadenoma and phyllodes tumors).