A recent study [37] which transfected and over-expressed murine ER-α in the epithelium of mouse mammary glands noted the rapid development of hyperplasias, which occasionally progressed to DCIS; this supports the notion that elevated ER-α may be partially responsible for the development of HELUs and their progression to more advanced lesions in humans. This evidence concerns the gene ESR1 and ductal breast carcinoma in situ.