RKIP cDNA encodes a protein of 186 amino acids, the sequence of which is 95% identical to the bovine 21- to 23-kD protein. The rat protein shows 85.5% identity with the human protein. highly expressed in the brain and associated with cytosolic proteins and small GTP-binding proteins. Multiple tissue Northern blots revealed the presence of a single mRNA in the different tissues of each species; a single band of 1.8 kb was identified in human, 1.45 kb in mouse, and 1.2 kb in rat tissues. The mRNA is particularly highly expressed in rat and mouse testis, where the level was 30 times higher than that in the brain. PBP is expressed strongly in the cell bodies of oligodendrocytes but only weakly elsewhere. Schwann cells and spinal nerve roots showed intense cytoplasmic PBP immunoreactivity.