Single Cell Immune and Non-immune Correlates of Response to Neoadjuvant Abemaciclib

This study is examining how the immune system responds to a treatment called abemaciclib (a CDK4/6 inhibitor) given before surgery in women with early-stage hormone receptor-positive, HER2-negative breast cancer. It is a research study collecting tissue samples — not testing a new drug. **You may be eligible if...** - You are post-menopausal (aged 55 or older, or have stopped menstrual periods for at least 12 months, or have confirmed hormonal post-menopausal status) - You have operable Stage I, II, or III breast cancer that is estrogen or progesterone receptor-positive and HER2-negative - Your breast tumor is at least 1 cm and accessible for biopsy before and after treatment **You may NOT be eligible if...** - You are pre-menopausal - You have HER2-positive or triple-negative breast cancer - You have other conditions that would prevent participation in the study procedures Talk to your doctor to see if this trial is right for you.