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Retinoic-acid receptor beta expression in melanocytes.

Retinoic acid receptors (RAR) and retinoic X receptors (RXR) are critical for skin homeostasis. In epidermis, RXRalpha and RARgamma isoforms are highly expressed but only weak or no expression of RARbeta has been reported. Here, we re-examined RARbeta in situ expression in comparison with that of the RXRalpha, in both normal skin and melanocytic tumours. In normal skin, RXRalpha was localized in epidermis, sebaceous glands and hair follicles, while RARbeta was detectable only in melanocytes and in stratum granulosum. RXRalpha was never detected in melanocytic tumours, neither in nevi, nor in melanomas. RARbeta was also absent from melanoma cells but was present in nevus cells. These results indicate that melanoma are characterised by simultaneous decrease of RARbeta and absence of RXRalpha that may be responsible for the RA-resistance of most melanoma cell lines.