The prohibitins, called PHB1 and PHB2, are highly conserved proteins that are present in nuclear and mitochondrial compartments in eukaryotic cells and exert multiple cellular functions (1). PHB2 (REA) was originally identified as an estrogen receptor (ER) specific coregulator. It directly interacts with hormone-bound ER and represses its transcriptional activity by competitive inhibition of SRC-1 coactivation of ER (2,3). Together with COUP-TFs, PHB2 also interacts with histone deacetylases HDAC1 and HDAC5, and mediates transcriptional regulation by ER through coupling the deacetylase to the transcription activation complex (4). Another important function of PHB2 is stablization of mitochondrial proteins. PHB1/PHB2 hetrodimers form large ring complexes on the mitochondrial membrane (5), acting as chaperones to stablize important mitochodrial proteins, such as OPA1 and Hax1 to support mitochondrial morphogenesis and protection against apoptosis (6-8).