Inclusion Criteria:

- Have a diagnosis of hormone receptor positive (HR+), human epidermal growth factor
receptor 2 negative (HER2-) breast cancer.

- To fulfill the requirement of HR+ disease, a breast cancer must express, by
immunohistochemistry, at least 1 of the hormone receptors (estrogen receptor or
progesterone receptor).

- To fulfill the requirement of HER2- disease, a breast cancer must not
demonstrate, at initial diagnosis or upon subsequent biopsy, overexpression of
HER2 by either immunohistochemistry or in-situ hybridization.

- Have recurrent, locally advanced, unresectable or metastatic breast cancer with
disease progression following anti-estrogen therapy.

- Have a performance status of 0 to 2 on the Eastern Cooperative Oncology Group scale.

- Have discontinued all previous therapies for cancer (including chemotherapy,
radiotherapy, immunotherapy, and endocrine therapy) for at least 21 days for
myelosuppressive agents or 14 days for nonmyelosuppressive agents prior to receiving
abemaciclib, and recovered from the acute effects of therapy (until the toxicity
resolves to either baseline or at least Grade 1) except for residual alopecia and
peripheral neuropathy.

- Have adequate organ function, including:

- Hematologic: absolute neutrophil count ≥1.5 × 10⁹/Liter (L), platelets ≥100 ×
10⁹/L, and hemoglobin ≥8 grams/deciliter. Participants may receive erythrocyte
transfusions to achieve this hemoglobin level at the discretion of the treating
physician. Initial treatment should not begin earlier than the day after the
erythrocyte transfusion.

- Hepatic: bilirubin ≤1.5 × the upper limit of normal (ULN) and alanine
aminotransferase ≤3 × ULN.

- Renal: serum creatinine ≤ULN.

- If a woman of child-bearing potential, must test negative for pregnancy at the time
of enrollment based on serum pregnancy test, and must agree to use a reliable method
of birth control during the program and for 3 months following the last dose of the
abemaciclib.

- If a man, must agree to use a reliable method of birth control and to not donate
sperm during the program and for at least 3 months following the last dose of
abemaciclib (or country requirements, whichever is longer).

- Are able to swallow capsules.

Exclusion Criteria:

- Are currently enrolled in a clinical trial involving an investigational product or
concurrently enrolled in any other type of medical research judged not to be
scientifically or medically compatible with this program.

- Have progressive central nervous system metastasis that require immediate local
therapy including (but not limited to) whole-brain radiotherapy, stereotactic
radiosurgery, surgical resection, or intrathecal chemotherapy.

- Have a history of progressive disease during prior therapy with a cyclin-dependent
kinase (CDK) 4&6 inhibitor.

- Have had major surgery within 14 days of the initial dose of abemaciclib.

- Have a personal history of any of the following conditions: syncope of either
unexplained or cardiovascular etiology, ventricular tachycardia, ventricular
fibrillation, or sudden cardiac arrest.

- Have active bacterial, fungal, and/or known viral infection.

- If lactating, must agree to not begin and/or discontinue breastfeeding.