The calcium-binding Protein S1A2 is a member of the S1 family of proteins containing 2 EF-hand calcium-binding motifs. S1 family genes are located as a cluster on chromosome 1q21, and S1 proteins consisting of at least 2 members are involved in the regulation of a number of cellular processes such as cell-cycle progression and cell differentiation. S1A2 was first detected in lung and kidney, and is mainly expressed in a subset of tissues and cells such as breast epithelia and liver. The S1A2 protein is a homodimer that undergoes a conformational change upon binding of calcium, and the active form functions in regulating cell proliferation and differentiation, gene transcription, and p53-dependent growth arrest and apoptosis. Accordingly, this protein is regarded as a putative tumor suppressor, and thus chromosomal rearrangements and reduced expression of S1A2 gene have been implicated in certain carcinomas.