A full-length cDNA encoding a 56 kDa liver protein recently implicated in the detoxification of acetaminophen (AP56) has been cloned by virtue of its similarity to the 56 kDa selenium-binding protein (SP56): in fact, the deduced AP56 amino acid sequence differs at only 14 residues from SP56. Isolation of genomic DNA recombinants from a Balb/c mouse cosmid genomic DNA library shows that SP56 and AP56 are encoded by two different genes. Using reverse transcription/PCR with oligonucleotide primers that distinguish the AP56 and SP56 mRNAs shows that the SP56 mRNA is highly expressed in liver, kidney and, to a lesser extent, lung; whereas the AP56 mRNA is mainly expressed in liver. Both mRNAs tend to be down-regulated in liver cell lines but remain high in DEN-induced liver tumours in vivo. The relevance of these findings is evaluated in terms of the postulated functions of the two proteins in mediating the anti-carcinogenic effects of selenium and detoxification mechanisms.