Abstract
Background: Keratinocytic tumors are commonest skin tumours. S100A6 is useful in differentiation of BCC, SCC and other adnexal tumours. We studied expression of S100A6 and its pattern to see its role in differentiation of various keratinocytic tumours.
Methods: Immunohistochemistry was performed using streptavidin-biotin peroxidase technique with the polyclonal antibody for S100A6. The expression was based on percentage of tumour cells showing cytoplasmic positivity and the cytoplasmic staining intensity. Only cytoplasmic expression was considered as positive.
Results: All the types of keratinocytic tumours showed positivity for S100A6 except BCCs. Among SCCs, higher expression of S100A6 was seen in well-differentiated SCCs as compared to poorly-differentiated ones. Only basosquamous BCCs i.e. BCCs with focal areas of squamous differentiation showed S100A6 positivity. In benign keratinocytic lesions including acanthomas and verrucas, mature squamous cells were found positive with no significant difference in pattern of staining. The precancerous lesion i.e. actinic keratosis and Bowen’s disease showed non-specific S100A6 staining.
Conclusions: S100A6 is consistently expressed in mature squamoid cells of keratinocytic tumours with a sensitivity of 98%, if we exclude BCCs.

Key words: S100A6, Squamous cell carcinoma, basal cell carcinoma, acanthoma, actinic keratosis, Bowen’s disease, verruca