The EF-hand protein S100A2 is a cell cycle regulator involved in tumori-genesis, acting through regulation of the p53 activation state. Metal ion-free S100A2 is homodimeric and contains two Ca 2+ -binding sites and two Zn 2+ -binding sites per subunit, whereby the Zn 2+ ion binding to one of the sites is coordinated by residues from two homodimers.