ERBB2 and extramammary Paget disease: Due to the rarity of the disease, our current knowledge about EMPD is based on a limited number of case series, and some controversies exist, such as pathogenic differences between primary and secondary EMPD, the optimal treatment for patients with recurrent EMPD, the factors affecting prognosis, and the clinical significance of human epidermal growth factor receptor 2 (HER2) overexpression.