ESR1 and neoplasm: Specifically, hierarchical clustering analyses indicated that the MMTV-PyMT mouse model is initially associated with the mature luminal signature characterized by the expression of luminal keratin 8/18, ER+, and PR+ at an early stage of tumor progression but progresses to luminal progenitor features characterized by basal-like tumors with low levels of ER in later stages, which is associated with metastasis [47].