A cDNA encoding a novel member of the cysteine proteinase family of proteins has been cloned from a human breast carcinoma cDNA library, by using a polymerase chain reaction-based cloning strategy. The isolated cDNA contains an open reading frame coding for a polypeptide of 321 amino acids that has been tentatively called cathepsin O. This protein presents all the structural features characteristic of the different cysteine proteinases identified to date, including the active site cysteine residue that is involved in covalent intermediate formation during peptide hydrolysis. The cathepsin O cDNA was expressed in Escherichia coli, and after purification and refolding, the recombinant protein was able to degrade the synthetic peptides benzyloxycarbonyl-Phe-Arg-7-amido-4- methylcoumarin and benzyloxycarbonyl-Arg-Arg-7-amido-4-methylcoumarin widely used as substrates for cysteine proteinases. Cathepsin O proteolytic activity was abolished by trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane (E-64), an inhibitor of this subclass of proteolytic enzymes, thus providing additional evidence that the isolated cDNA codes for an authentic cysteine proteinase. Northern blot analysis of poly(A)+ RNAs isolated from a variety of human tissues demonstrated that cathepsin O is expressed in all examined tissues, which is consistent with a putative role of this protein as a proteolytic enzyme involved in normal cellular protein degradation and turnover.