Calretinin is a 29 kDa calcium-binding protein involved in calcium signalling. It is found abundantly in neurons and the thymus, and is associated with the kinetocore during the cell cycle. Outside the nervous system, calretinin is found in a number of cells with varying expression, including mesothelial cells, steroid producing cells, testicular cells, ovarian surface epithelium, some neuroendocrine cells, breast glands, and hair follicle cells. Diagnostically, calretinin is used as a positive marker for both benign mesothelium and in malignant mesothelioma (Saydan et al). It’s use in the identification of mesothelioma in cytological preparations (Doglioni et al), and in the differential diagnosis between malignant mesothelioma and adenocarcinoma in FFPE cell blocks of cytological fluids, washings and aspirates (Wiezorekd and Krane) have been described. Calretinin can also be used to help differentiate lung tumours (Marchevsky), and also to distinguish between different types of brain tumour, i.e. neuronal rather than glial differentiation.