Atypical ductal hyperplasia is an atypical proliferative lesion that falls in between the continuum from normal hyperplasia to low grade ductal carcinoma in situ. The disease is atypical ductal hyperplasia (EFO_0008491, an Experimental Factor Ontology identifier). Also known as: DIN 1B, atypical breast ductal hyperplasia, atypical ductal breast hyperplasia, atypical ductal hyperplasia of breast, atypical ductal hyperplasia of the breast, ductal intraepithelial neoplasia, Grade 1B.