The monoclonal antibody clone 203 recognizes S100 calcium-binding protein A12 (S100A12). The antibody does not cross-react with family members S100A7, S100A8 and S100A9 and S100A8/A9 heterodimers. S100A12, also known as calgranulin C, is a member of the S100 protein family. To date, the family consists of 22 members. S100 proteins are low molecular weight calcium binding proteins. The proteins consist of 2 calcium binding EF-hands located on the termini flanked by hydrophobic hinge regions. Calcium ions acts as a global second messenger involved in the regulation of many aspects of cell function. S100 proteins are involved in sensing and transforming calcium signals to downstream cellular responses. S100 proteins are located in the cytoplasm and/or nucleus of a wide range of cell types and involved in the regulation of cellular processes such as cell cycle progression and differentiation and inflammatory responses. Several S100 members have been found to act as cytokines in inflammation, particularly in autoimmune skin conditions such as psoriasis. S100A12 is involved in calcium-dependent signal transduction pathways and has regulatory effects on cytoskeletal components thereby modulating neutrophil activities. Studies have shown that S100A12 might have potential as a biomarker for arthritis since their levels correlate with disease activity and are able to predict relapse and response to anti-inflammatory therapy. Unfortunately, studying the biology of S100A12 and other calgranulins is challenging due to its tendency to form complexhomodimer, heterodimer and heterotetramers structures.