Endocrine Therapy With or Without Abemaciclib (LY2835219) Following Surgery in Participants With Breast Cancer

Inclusion Criteria: * Women (regardless of menopausal status) or men ≥18 years of age (or per local regulations). * The participant has confirmed HR+, HER2-, early stage resected invasive breast cancer without evidence of distant metastases. * The participant must have undergone definitive surgery of the primary breast tumor. * The participant must have tumor tissue from breast (preferred) or lymph node for exploratory biomarker analysis available prior to randomization. * Pathologic lymph node involvement and at least one of the following indicating a higher risk of recurrence: * 4 or more positive axillary lymph nodes * Tumor size of at least 5 centimeters * Grade 3 defined as at least 8 points on the Bloom Richardson grading system * Ki-67 index by central analysis of ≥20% on untreated breast tissue * The participant must be randomized within 16 months from the time of definitive breast cancer surgery. * The participant may receive up to 12 weeks of endocrine therapy until randomization following the last non-endocrine therapy (surgery, chemotherapy, or radiation) whichever is last. * Participants must have recovered (grade ≤1) from the acute effects of chemotherapy and radiotherapy and from surgical side effects following definitive breast surgery. * Women of reproductive potential must have a negative blood pregnancy test and agree to use highly effective contraceptive methods. * The participant has a Eastern Cooperative Oncology Group (ECOG) performance status…