Calretinin is an intracellular calcium-binding protein of 31.5 KD belonging to the troponin C superfamily characterised by a structural motif described as the EF-hand domain. Calcium is an important moderator of a number of vital physiological processes, including neuronal excitability, axonal transport, synthesis and release of some neurotransmitters, membrane permeability and enzym e activity. Calretinin possesses six characteristic EF-hand structures of which five are able to bind calcium ions with high affinity. Calretinin is found in the nervous system and thymus. Calretinin can also be demonstrated at precise moments of the cell cycle, in particular, G1 phase and mitosis, and is associated with kinetochore microtubules which may indicate a role in chromosome separation. The imunohistochemical detection of calretinin in developing cerebellum is restricted to the later stages indicated by weak staining from week 21 of gestation, in Purkinje and basket cells and in neurons of the dentate nucleus. This intensity of staining increases as the cerebellum matures. Other related calcium -binding proteins such as calbindin D28 and parvalbumin are expressed at earlier stages of development. In tumours, calretinin has been detected in mesotheliomas and some pulmonary adenocarcinomas.