During the development of an organism cell proliferation, differentiation and cell death are tightly balanced, and are controlled by a number of different regulators. Alterations in this balance are often observed in a variety of human diseases. The role of Ca²⁺ as one of the key regulators of the cell is discussed with respect to two recently discovered proteins, ALG-2 and AIP, of which the former is a Ca²⁺-binding protein, and the latter is substrate to various kinases. The two proteins interact with each other in a Ca²⁺-dependent manner, and the role of the complex ALG-2/AIP as a possible modulator at the interface between cell proliferation and cell death is discussed.