ESR1 and ductal breast carcinoma in situ: Today, the standard method used to differentiate difficult cases on the borderline between low grade DCIS and UDH is to perform immunohistochemistry (IHC) for estrogen receptor (ER) and basal cytokeratins (CK5/6), with low grade DCIS tending to show the pattern of ER positive and CK5/6 negative, while UDH shows a mosaic-staining pattern for CK5/6 and variable ER positivity [43].