hCDC47 is a human member of the MCM family, which has been implicated in the regulatory machinery causing DNA to replicate once per cell cycle. We examined its protein expression and localization in normal human tissues, using immunostaining with polyclonal antibodies. Positive nuclei were found in the proliferative components of lymph nodes, bone marrow, epidermis and mucosa. Immunohistochemical analysis was also performed for 3 types of cutaneous keratinocytic tumor originating from same cell type but showing different grades of malignancy. In seborrheic keratosis, a benign condition, cells with hCDC47-positive nuclei were located in the outermost layers of the tumor lobules, while in Bowen's disease, carcinomas in situ and squamous-cell carcinomas, they were present throughout the lesions. The percentages of hCDC47-positive cells were 65.4% in squamous-cell carcinomas, 60.9% in Bowen's disease, 12.6% in seborrheic keratosis and 3.9% in normal epidermis (n = 5 in all cases). Further expansion of the analysis to include malignant tumors from several other organs revealed that all malignant lesions tested contained more nuclear hCDC47-positive cells than their normal counterparts. Our findings indicate that hCDC47 plays a role in normal and neoplastic cell growth in vivo and that hCDC47 immunolocalization could be used as an index of cell proliferation in tissue sections.