Squamous cell carcinoma antigens (SCCA) 1 and 2 are inhibitory members of the high-molecular-weight serine proteinase inhibitor (serpin) family. The biological functions of SCCA1 and 2 are unknown. One approach to determining the function of human proteins is to study orthologs in other species, such as the mouse. The purpose of this study was to determine whether orthologs to human SCCA1 or 2 exist in the mouse. We report the identification and characterization of a novel serpin, sqn5 (now designatedScca2). Comparative amino acid sequence analysis suggests thatScca2is a member of the ov-serpin subfamily of serpins with highest homology to SCCA1 and SCCA2. Fluorescencein situhybridization revealed that theScca2mapped nearBcl2on mouse chromosome 1. This region is syntenic with the human locus for SCCA1 and SCCA2 on 18q21.3. The tissue expression patterns as determined by RT-PCR showed a restricted distribution.Scca2was detected in the lung, thymus, skin, and uterus, as are SCCA1 and SCCA2. Unlike the SCCAs, however,Scca2was detected also in the gastrointestinal tract. Enzyme-inhibition assays using a GST–SCCA2 fusion protein revealed that SCCA2 inhibited chymotrypsin-like serine proteinases, but not papain-like cysteine proteinases. SCCA2 inhibited CTSG at 1:1 stoichiometry and with a second-order rate constant ofk_ass= 1.7 × 10⁵M⁻¹s⁻¹. SCCA2 also inhibited human mast cell chymase but the stoichiometry was 2:1, and the second-order rate constant wask_ass= 0.9 × 10⁴M⁻¹s⁻¹. This inhibitory profile is identical to that observed for human SCCA2. Based on these findings,Scca2appears to be the murine ortholog of human SCCA2.