PBR (Peripheral type Benzodiazepine Receptor) is a ubiquitous protein (18 kDa) found in most steroidogenic tissues. It is located primarily in the outer mitochondrial membrane of steroid producing tissues and is found in association with a 34 kDa voltage dependent anion channel protein (VDAC). PBR interacts with some benzodiazepines and has different affinities than its endogenous counterpart. PBR appears to be a key factor in the transport of cholesterol from the outer to the inner mitochondrial membrane to permit the initiation of steroid hormone synthesis. It is thought to be part of the mitochondrial permeability transition pore (PT) and to be involved in apoptosis. It is speculated that patients with congenital lipoid adrenal hyperplasia, who cannot make any steroids, might have a genetic lesion in BZRP. A short form, PBRS is also expressed in the same tissues, but at a level about ten times that of PBR.