Abstract: JIANG Zheng-qiang*, MAN Xiao-yong, ZHENG Min. *Department of Dermatology, Second People′s Hospital of Linhai, Linhai 317016, Zhejiang, China Corresponding author: MAN Xiao-yong, Email: manxyzju@gmail.com 【Abstract】 Objective To investigate the expression and significance of keratin 17 （K17） in proliferative skin diseases, including psoriasis, basal cell carcinoma （BCC）, squamous cell carcinoma （SCC） and malignant melanoma. Methods Tissue specimens were collected from the lesions of 14 patients with severe plaque-type psoriasis, 16 patients with BCC, 16 patients with SCC, 8 patients with malignant melanoma, as well as from the normal skin of 17 patients with trauma. Immunohistochemistry was conducted to detect the expression of K17 in these tissue samples. Results K17 was absent in the cytoplasm of keratinocytes in the basal layer, prickle cell layer or granular layer of the normal skin. There was a strong expression of K17 in the prickle cell layer, but a weak or negative expression of K17 in the basal layer of psoriatic skin, and parakeratotic cells did not express K17. In BCC tissues, K17 was absent in carcinoma cells, but visible in peritumoral cells in the prickle cell layer and granular layer. In SCC tissues, K17 was localized in highly differentiated carcinoma cells, but not in lowly differentiated carcinoma cells. There was a strong expression of K17 throughout the epidermis above the melanoma, but a negative expression in the melanoma cells or melanocytes. Conclusion K17 may serve as a molecular marker for the differential diagnosis of some proliferative skin diseases.