A novel cDNA encoding a cysteine protease of the papain family named cathepsin X was obtained by PCR amplification from a human ovary cDNA library. The cathepsin X cDNA is ubiquitously expressed in human tissues and contains an open reading frame of 912 nucleotides encoding a predicted protein of 303 amino acids. All highly conserved regions in papain-like cysteine proteases including the catalytic residues are present in cathepsin X. The mature part of cathepsin X is 26-32% identical to human cathepsins B, C, H, K, L, O, S and W. The cathepsin X sequence contains several unique features: (i) a very short proregion; (ii) a three amino acid residue insertion in a highly conserved region between the glutamine of the putative oxyanion hole and the active site cysteine; and (iii) a second insertion of 15 amino acid residues that can be aligned with the occluding loop region in cathepsin B.