Familial disseminated plaque type porokeratosis with multiple horns and squamous cell carcinoma involving anal skin.

Porokeratosis is a disorder of keratinization showing a well-defined lesion with a hyperkeratotic ridge on the border that contains the coronoid lamella. We report familial (autosomal dominant with reduced penetrance) disseminated plaque type (Mibelli's type) porokeratosis in a father and son. In the father, there were multiple horns and a large squamous cell carcinoma in a large lesion over the perianal region that reached up to the squamo-columnar junction of the anal mucosa and even invaded the anal sphincteric muscles. Disseminated lesions of the Mibelli's type, development of horns, and malignancy in this unusual location have not been previously reported.