PIK3CA and extramammary Paget disease: Frequent gene alternations in ERBB2, RAS, RAF, AKT1, and PIK3C in EMPD lesions detected in previous studies suggested that HER2, which is encoded by ERBB2, and its downstream signaling, including RAS/RAF-MEK-ERK pathway and PI3K-AKT-mTOR pathway, may play important roles in the pathogenesis of EMPD in many cases.