ERBB2 and Decreased total neutrophil count: However, a recent study of HR+, HER2− metastatic breast cancer patients receiving palbociclib-based therapy reported a similar prevalence of adverse effects to treatment and minimal differences in prevalence of specific adverse effects such as neutropenia, diarrhea, and fatigue between patients <70 and patients ≥70 over the course of the first six months of treatment [48].