Inclusion Criteria:

          1. Men or women at least 18 years of age with histologically or cytologically confirmed
             adenocarcinoma of the breast with unresectable or metastatic disease.

          2. Most recent tumor biopsy or surgical resection specimen must be either
             estrogen-receptor (ER) positive, progesterone receptors (PgR) positive, or both, as
             defined by immunohistochemistry (IHC) ≥1% (as per the American Society of Clinical
             Oncology (ASCO)-College of American Pathologists (CAP) guidelines).

          3. HER2-negative breast cancer defined as a negative in situ hybridization test or an
             immunohistochemistry (IHC) status of 0, 1+ or 2+. If IHC is 2+ (i.e. indeterminate), a
             negative in situ hybridization (Fluorescent in situ hybridization (FISH), Chromogenic
             in situ hybridization (CISH), or Silver-enhanced in situ hybridization (SISH)) test is
             required by local laboratory testing. (as per the ASCO-CAP guidelines).

          4. Postmenopausal status or receiving ovarian ablation with a GnRH agonist such as
             goserelin. Postmenopausal status is defined by any one of the following criteria:

               -  Prior bilateral oophorectomy.

               -  Age ≥60 years.

               -  Age <60 and amenorrhea for 12 or more months (in the absence of chemotherapy,
                  tamoxifen, toremifen, or ovarian suppression) and FSH, LH, and estradiol in the
                  postmenopausal range per local normal If the patient does not meet criteria for
                  postmenopausal status but is receiving ovarian ablation therapy with a
                  gonadotropin-releasing hormone (GnRH) agonist such as goserelin, the patient is
                  eligible for this study, provided that the GnRH agonist is started at least 2
                  weeks prior to C1D1 of anti-estrogen therapy.

          5. Have evidence of measurable or unmeasurable disease.

          6. Eastern Cooperative Group (ECOG) performance status of 0 or 1.

          7. No prior cdk 4/6 inhibitor (Closed to Accrual). If patient has not previously received
             letrozole, letrozole will be supplied by Novartis. If previously progressed on
             letrozole, another aromatase inhibitor that the patient has not previoulsy received is
             allowed, per standard of care (anastrazole or exemestane, not supplied by study).
             Ribociclib will be supplied by Novartis. If patient has previously received letrozole,
             anastrazole, and exemestane, (s)he is not eligible. For scenario 1, patients are
             allowed to have started the aromatase inhibitor within 4 consecutive weeks prior to
             protocol registration. For instance, it is acceptable for patient who will be treated
             with letrozole in scenario #1, to have started letrozole within 4 consecutive weeks
             prior to protocol registration. No prior fulvestrant allowed.

          8. Scenario 2: the patient must have received an aromatase inhibitor (letrozole,
             arimidex, exemestane) or tamoxifen or fulvestrant plus palbociclib as standard of care
             or received a CDK4/6 inhibitor (palbociclib or ribociclib or abemaciclib), and
             demonstrated evidence of disease progression. If the patient was enrolled in a
             randomized clinical trial involving ribociclib or abemaciclib or palbociclib (such as
             the MONALEESA or PALOMA series of trials), then it must be known after study
             discontinuation and unblinding that the patient received the investigational drug and
             not placebo. Ribociclib or abemaciclib or palbociclib can also be given as standard of
             care. Documentation of progression and duration of response on aromatase inhibitor or
             tamoxifen plus CDK 4/6 inhibitor should be provided whenever possible. If patient
             received prior fulvestrant, exemestane must be the hormone therapy backbone in the
             randomization. If patient received prior exemestane, fulvestrant must be the hormone
             therapy backbone in the randomization. If neither has been administered, selection of
             fulvestrant or exemestane in the randomization will be per investigator discretion.

          9. Adequate baseline laboratory studies (hematologic and chemistry), including the
             following parameters:

               -  Absolute neutrophil count ≥ 1500 per microliter, Platelets ≥ 75,000 per
                  microliter, Hemoglobin level ≥ 8.0 gm/dL on screening complete blood count.

               -  Potassium, sodium, total calcium (corrected only in the case of hypoalbuminemia),
                  magnesium, and phosphorus within normal limits of the local laboratory (screening
                  values can be rechecked after electrolyte repletion and before the first dose of
                  study medication, if necessary).

               -  Serum creatinine level ≤ 1.5 mg/dL or estimated glomerular filtration rate > 50
                  mL/min.

               -  In absence of liver metastases, alanine aminotransferase (ALT) and aspartate
                  aminotransferase (AST) should be below 2.5 × the upper limit of normal (ULN). If
                  the patient has liver metastases, ALT and AST should be < 5 × ULN.

               -  Total bilirubin ≤ 1.5 x ULN. (In patients with well documented Gilbert's
                  Syndrome, total bilirubin ≤ 3 × ULN with direct bilirubin within normal range.)

               -  international normalized ratio (INR) ≤ 1.5

         10. a) Written informed consent and HIPAA authorization obtained from the subject/legal
             representative prior to performing any protocol-related procedures b) Subjects must be
             willing and able to comply with scheduled visits, treatment schedule, laboratory
             testing, and other requirements of the study

         11. Must be able to swallow ribociclib and oral aromatase inhibitor, such as letrozole or
             exemestane.

        Exclusion Criteria:

          1. Patient has a known hypersensitivity to any of the excipients of ribociclib, aromatase
             inhibitors (such as letrozole) or fulvestrant

          2. Active central nervous system (CNS) disease. History of CNS metastases or cord
             compression is allowable if the patient has been clinically stable for at least 4
             weeks since completion of definitive treatment and is off systemic steroids. In the
             case of brain metastases, the patient must have stable or improved imaging at least 4
             weeks after completion of definitive treatment. If there is evidence of active
             leptomeningeal disease, the patient is ineligible.

          3. Identified as having visceral crisis, lymphangitic spread, or leptomeningeal
             carcinomatosis. Visceral crisis is not the mere presence of visceral metastases but
             implies severe organ dysfunction as assessed by symptoms and signs, laboratory
             studies, and rapid progression of the disease.

          4. Received more than 1 prior systemic chemotherapy in the unresectable or metastatic
             setting. If the patient received 1 prior systemic chemotherapy, the patient is
             eligible. Having received prior therapies for breast cancer (such as everolimus or
             experimental agents) does not affect eligibility for this study.

          5. Completion of major surgery, chemotherapy, targeted therapy (such as everolimus or
             experimental agents_ or radiation within 14 days prior to starting investigational
             drug or has not recovered from major side effects. There is no required washout period
             from completion of prior anti-estrogen therapy (either scenario) or prior CDK 4/6
             inhibitor (if scenario 2) to initiation of ribociclib/placebo and anti-estrogen on
             trial.

          6. Residual acute toxic effects of prior anti-cancer therapy that have not resolved to
             CTCAE v.4 Grade ≤1. Exception to this criterion: patients with grade 1 taxane-induced
             neuropathy, any grade of alopecia, amenorrhea or other toxicities not considered a
             safety risk for the patient as per investigator's discretion, are allowed to enter the
             study.

          7. Presence of a concurrent malignancy or malignancy diagnosed within 5 years of
             randomization, with the exception of basal or squamous cell carcinoma,
             non-melanomatous skin cancer, curatively resected cervical cancer, localized prostate
             cancer treated with curative intent, and stage I colorectal cancer treated with
             curative resection.

          8. Patient has impairment of gastrointestinal (GI) function or GI disease that may
             significantly alter the absorption of the study drugs (e.g. ulcerative diseases,
             uncontrolled nausea, vomiting, diarrhea, malabsorption syndrome, or small bowel
             resection).

          9. Patient has a known history of HIV infection (testing not mandatory).

         10. Clinically significant, uncontrolled heart disease and/or cardiac repolarization
             abnormality including any of the following:

               -  History of myocardial infarction (MI), angina pectoris, symptomatic pericarditis,
                  or coronary artery bypass graft (CABG) within 6 months prior to study entry

               -  Documented cardiomyopathy

               -  Patient has a known Left Ventricular Fraction (LVEF) <50% as determined by
                  Multiple Gated acquisition (MUGA) scan or echocardiogram (ECHO).

               -  Long QT syndrome or family history of long QT syndrome or family history of
                  idiopathic sudden death or congenital long QT syndrome, or any of the following:

                  i. Risk factors for Torsades de Pointe (TdP) including uncorrected hypocalcemia,
                  hypokalemia or hypomagnesaemia, history of cardiac failure, or history of
                  clinically significant/symptomatic bradycardia ii. Concomitant medications(s)
                  with a known risk to prolong the QT interval and/or known to cause Torsades de
                  Pointe that cannot be discontinued or replaced by safe alternative medication
                  (e.g. within 5 half-lives or 7 days prior to starting study drug) iii. Inability
                  to determine the QTc interval

               -  Clinically significant cardiac arrhythmias (e.g., ventricular tachycardia),
                  complete left bundle branch block, high-grade AV block (e.g., bifascicular block,
                  Mobitz type II and third degree AV block)

               -  Systolic Blood Pressure (SPB) >160 or <90 mmHg.

         11. Corrected QT interval (QTcF) > 450 msec (QT interval using Fridericia's correction) on
             screening electrocardiogram. If QTc prolongation is felt to be related to electrolyte
             imbalance, an EKG can be repeated after correction of electrolytes. Mean resting heart
             rate 50-100 bpm (determined from ECG).

         12. The presence of any other concurrent severe and/or uncontrolled medical condition that
             would, in the investigator or treating physician's judgment, cause unacceptable safety
             risks, contraindicate patient participation in the clinical study or compromise
             compliance with the protocol. This includes uncontrolled infections that could
             potentially be exacerbated by anti-neoplastic treatment, active untreated or
             uncontrolled fungal bacterial or viral infections, etc.

         13. Currently receiving treatment, including medications and herbal preparations, with
             known strong inducers or inhibitors of cytochrome p450 enzymes CYP3A4/5 medications
             that have a narrow therapeutic window and are predominately metabolized through
             CYP3A4/5 or herbal preparations/medications, dietary supplements, which cannot be
             discontinued prior to receiving investigational drug. Anti-retrovirals,
             anti-microbials, and anti-arrhythmics are the most common medications that interact
             with these enzyme. Please refer to Section 5: Pharmaceutical Information and Appendix
             B for more information and a list of drugs that should not be used concurrently with
             ribociclib.

         14. Patients who are receiving any other investigational agents concurrently or have
             received investigational agents within 14 days or 5 half-lives of the compound or
             active metabolites, whichever is longer before the first dose of the study treatment.

        15 Patient is concurrently using hormone replacement therapy. 16. Subject is pregnant or
        nursing. Serum or urine Beta-Human chorionic gonadotropin (HCG) must be checked in all pre-
        and peri-menopausal patients. (Fulvestrant is pregnancy category D and CDK4/6 inhibitors
        have demonstrated teratogenicity/fetotoxicity in animal studies.)