Serpin B3 (SERPINB3)

Squamous cell carcinoma antigen (SCCA), a member of the serine protease inhibitor (serpin) family, is a subfraction of the tumor associated antigen TA-4, is a glycoprotein, with a molecular weight between 42 to 48 kDa. 

Total SCCA in the circulation comprises 2 nearly identical, approximately 45 kDa proteins, SCCA1 and SCCA2. Both proteins are members of the high-molecular weight serine proteinase inhibitor (serpin) family with SCCA1 paradoxically inhibiting lysosomal cysteine proteinases and SCCA2 inhibiting chymotrypsin-like serine proteinases.

Although SCCA1 and SCCA2 are detected in the cytoplasm of normal squamous epithelial cells, neither serpin is detected normally in the serum.