Protein S1-A11, also known as S1 calcium-binding protein A11, S1A11 and MLN7, is a member of theS-1 family. S1A11 is widely expressed in multiple tissues, and is located in cytoplasm, nucleus, and even cell periphery. S1A11 exists as a non-covalent homodimer with an antiparallel conformation. Ca(2+) binding to S1A11 would trigger conformational changes which would expose the hydrophobic cleft of S1A11 and facilitate its interaction with target proteins. As a dual cell growth mediator, S1A11 acts as either a tumor suppressor or promoter in many different types of tumors and would play respective roles in influencing the proliferation of the cancer cells. In the nucleus, S1A11 suppresses the growth of keratinocytes through p21 (CIP1/WAF1) activation and induces cell differentiation. S1A11 is also a novel diagnostic marker in breast carcinoma.