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 ≤1.
* The participant has adequate organ function.
* The participant is able to swallow oral medications.

Exclusion Criteria:

* Metastatic disease (including contralateral axillary lymph nodes) or node-negative disease.
* Participants with inflammatory breast cancer.
* Participants with a history of previous breast cancer, with the exception of ipsilateral ductal carcinoma in situ (DCIS) treated by locoregional therapy alone ≥5 years ago. Participants with a history of contralateral DCIS treated by local regional therapy at any time may be eligible. Participants with a history of any other cancer (except non-melanoma skin cancer or carcinoma in situ of the cervix), unless in complete remission with no therapy for a minimum of 5 years from the date of randomization are excluded.
* Females who are pregnant or lactating.
* The participant has previously received treatment with any CDK4 and CDK6 inhibitor.
* The participant is receiving concurrent exogenous reproductive hormone therapy (for example, birth control pills, hormone replacement therapy, or megestrol acetate).
* The participant has previously received endocrine therapy for breast cancer prevention (tamoxifen or aromatase inhibitors) or raloxifene.
* The participant has serious preexisting medical condition(s) that, in the judgment of the investigator, would preclude participation in this study.
* The participant has a personal history of any of the following conditions: syncope of cardiovascular etiology, ventricular arrhythmia of pathological origin or sudden cardiac arrest. Any participant with a history of venous thromboembolism (VTE).
* The participant has active systemic infections or viral load.
* The participant has received an experimental treatment in a clinical trial within the last 30 days or 5 half-lives, whichever is longer.