 EIN lesions have been discovered by a combination of molecular, histologic, and clinical outcome studies beginning in the 1990s which provide a multifaceted characterization of this disease.They are a subset of a larger mixed group of lesions previously called "endometrial hyperplasia".The EIN diagnostic schema is intended to replace the previous "endometrial hyperplasia" classification as defined by the World Health Organization in 1994, which have been separated into benign (benign endometrial hyperplasia) and premalignant (EIN) classes in accordance with their behavior and clinical management.