Recent data have shown that expression of the cytoplasmic low-molecular-weight isoforms (LMW) of cyclin E is associated with resistance to aromatase inhibitors41 and to palbociclib in vitro.42 Similarly, overexpression of LMW cyclin E detected by immunohistochemistry has been associated with a lesser benefit from treatment with palbociclib and endocrine therapy in patients with metastatic BC.42 In our WB studies of the PDR models, LMW isoforms of cyclin E1 were not detected, suggesting that the full length is predominantly overexpressed in our models of acquired resistance. This evidence concerns the gene CYP19A1 and breast cancer.