TFAM is a mitochondrial transcription factor that is a key activator of mitochondrial transcription as well as a participant in mitochondrial genome replication. TFAM binds mitochondrial promoter DNA to aid transcription of the mitochondrial genome. Studies in mice have demonstrated that this protein is required to regulate the mitochondrial genome copy number and is essential for embryonic development. A mouse model for Kearns–Sayre syndrome was produced when expression of this protein was eliminated by targeted disruption in heart and muscle cells.