CDK4 and Pancytopenia: However, cases have been described in the literature in which patients received continuous endocrine therapy and CDK4/6 inhibitors and achieved a sufficient, long-lasting response, e.g. a 46-year-old premenopausal patient who underwent a bone marrow aspiration for pancytopenia, which revealed infiltration by a poorly differentiated HR + /HER2- lobular breast carcinoma.