Serpins are the largest and most diverse family of serine protease inhibitors which are involved in a number of fundamental biological processes such as blood coagulation, complement activation, fibrinolysis, angiogenesis, inflammation and tumor suppression and are expressed in a cell-specific manner. Serpins are a group of proteins with similar structures that were first identified as a set of proteins able to inhibit proteases. The acronym serpin was originally coined because many serpins inhibit chymotrypsin-like serine proteases (serine protease inhibitors). Over 1000 serpins have been identified. Mouse SerpinB3, also known as Squamous cell carcinoma antigen 1, SCCA-1, SERPINB3, SCCA and SCCA1, is a cytoplasm protein which belongs to the serpin family and Ov-serpin subfamily. SerpinB3 may act as a protease inhibitor to modulate the host immune response against tumor cells. Mouse SerpinB3a and SerpinB3b, but not Serpinb3c, are functional, inhibiting both serine and cysteine proteinases with different inhibitory profiles due to the difference of two amino acids in their reactive site loops. SerpinB3a is ubiquitously expressed in most tissues, whereas expression of SerpinB3b is limited to keratinocytes. SerpinB3a and SerpinB3b may play different roles by inhibiting intrinsic or extrinsic proteinases with different expression distributions and different inhibitory profiles.