A 24 kDa protein from rat liver peroxisomal membrane was isolated and subjected to Edman degradation. Using the N-terminal sequence of this polypeptide we have identified several rat and human expressed sequence tags in the GenBank Database. The complete sequence of a human cDNA clone was determined. The open reading frame encodes an extremely basic protein 212 amino acid residues long. A high similarity between this mammalian protein and hypothetical proteins from Caenorhabditis elegans and Neurospora crassa was found. Hydropathy analysis reveals the existence of two putative membrane-spanning domains in conserved regions of the three homologous proteins.