S100 Calcium Binding Protein A1, also known as S100A1, belongs to a family of proteins characterized by the presence of two EF-hand calcium binding motifs.1 S100 proteins are found in a diverse array of cells and are localized within the cytoplasm and/or nucleus. The reactivity of anti-S100A1 is observed in normal renal tissue, including proximal tubules, loops of Henle, collecting ducts, as well as in skeletal muscle and dendritic cells located in the lymph nodes of normal tissues. In the context of immunohistochemistry, anti-S100A1 has been reported to be effective in distinguishing renal oncocytoma from chromophobe renal cell carcinoma.