Calretinin is a calcium-binding protein related to calmodulin and calbindin. It is found in neuron groups in the cerebral cortex, retina and spinal cord. Calretinin is produced increasingly in the tumour cells of a mesothelioma and serves immunohistologically as an important diagnostic agent. In normal skin, calretinin is expressed in the inner cell layer of the outer root sheath of the harfollicle and in the secretory and ductal parts of the eccrine but not in the apocrine sweat glands. Furthermore, calretinin can be detected immunohistologically in the ductal and secretory parts of the sebaceous glands. The following adnexal tumors express calretinin: trichilemmomas, tumors of the follicular infundibulum and some (infundibulocystic) basal cell carcinomas.
Spiradenomas, apocrine hidradenomas, hidradenomas and syringocystadenoma papilliferum are negative for calretinin.
Eccrine mixed tumors (see below adnexal tumors of the skin) may show a strong reactivity (differentiation towards eccrine gland functions).