S100A1 is a member of the S100 family of proteins that contains 2 EF-hand calcium-binding motifs and that is involved in the regulation of a number of cellular processes such as cell cycle progression and differentiation. It is expressed in the cytoplasm and/or nuclei of cardiac and skeletal muscle as well as neuronal cells. S100A1 is useful in distinguishing between renal cell carcinomas (protein is present) and chromophobe renal cell carcinomas (protein is absent). It is also a specific and sensitive marker for nephrogenic adenoma.