In vertebrates, the prion protein (PrP, gene name Prnp) is expressed in diverse cell types and tissues, especially in the brain. In the adult brain, PrP contributes to homeostasis. Here, proteins expressed in the adult mouse brain which coimmunoprecipitated with PrP via the Fab antibody D18 were identified by comparison to PrP-specific immunoprecipitates from Prnp knockout brain.