Inclusion Criteria:

          -  Male or female advanced (locoregionally recurrent or metastatic) breast cancer not
             amenable to curative therapy.

          -  In the case of women, both pre/perimenopausal and postmenopausal patients are allowed
             to be included in this study; menopausal status is relevant for the requirement of
             goserelin to be used concomitantly with ribociclib and letrozole.

               1. Postmenopausal status is defined either by:

                  I). Prior bilateral oophorectomy OR ii). Age ≥ 60 OR iii). Age < 60 and
                  amenorrhea for 12 or more months (in the absence of chemotherapy, tamoxifen,
                  toremifen, or ovarian suppression) and FSH and estradiol in the postmenopausal
                  range per local normal range. If patient is taking tamoxifen or toremifene and
                  age < 60, then FSH and plasma estradiol levels should be in post-menopausal range
                  per local normal range (NCCN Guidelines version 2.2017).

                  Note: For women with therapy-induced amenorrhea, serial measurements of FSH
                  and/or estradiol are needed to ensure menopausal status.

               2. Premenopausal status is defined as either:

                  I). Patient had last menstrual period within the last 12 months, OR ii). If on
                  tamoxifen or toremifene within the past 14 days, plasma estradiol and FSH must be
                  in the premenopausal range per local normal range, OR iii). In case of therapy
                  induced amenorrhea, plasma estradiol and/or FSH must be in the premenopausal
                  range per local normal range.

               3. Perimenopausal status is define as neither premenopausal nor postmenopausal Note:
                  Throughout this document, perimenopausal and premenopausal status is grouped
                  together and referred as "Premenopausal"

          -  Patient has a histologically and/or cytologically confirmed diagnosis of
             estrogen-receptor positive and/or progesterone receptor positive breast cancer by
             local laboratory.

          -  Patient has HER2-negative breast cancer defined as a negative in situ hybridization
             test or an IHC status of 0, 1+ or 2+. If IHC is 2+, a negative in situ hybridization
             (FISH, CISH, or SISH) test is required by local laboratory testing.

          -  Patient has an Eastern Cooperative Oncology Group (ECOG) performance status ≤2

          -  Patient has adequate bone marrow and organ function as defined by ALL of the following
             laboratory values (as assessed by local laboratory):

               -  Absolute neutrophil count ≥ 1.5 × 10^9/L

               -  Platelets ≥ 100 × 10^9/L

               -  Hemoglobin ≥ 9.0 g/dL

               -  Potassium, sodium, calcium corrected for serum albumin and magnesium within
                  normal limits or corrected to within normal limits with supplements before first
                  dose of the study medication

               -  INR ≤1.5

               -  Serum creatinine <1.5 mg/dl or creatinine clearance≥50 mL/min

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

               -  Total serum bilirubin < ULN; or total bilirubin ≤ 3.0 × ULN with direct bilirubin
                  within normal range in patients with well-documented Gilbert's Syndrome

          -  Patient must have a 12-lead ECG with ALL of the following parameters at screening:

               -  QTcF interval at screening <450 msec (using Fridericia's correction)

               -  Resting heart rate ≥ 50 bpm

        Exclusion Criteria:

          -  Patient who received any CDK4/6 inhibitor

          -  Patient who received any prior systemic hormonal therapy for advanced breast cancer;
             no more than one prior regimen of chemotherapy for the treatment of metastatic disease
             is permitted

        Note:

          -  Patients who received (neo) adjuvant therapy for breast cancer are eligible. If the
             prior neo (adjuvant) therapy included letrozole or anastrozole the disease free
             interval must be greater than 12 months from the completion of treatment until study
             entry.

          -  Patients who received ≤ 28 days of letrozole or anastrozole for advanced disease prior
             to inclusion in this trial are eligible.

          -  Any prior (neo) adjuvant anti-cancer therapy or prior chemotherapy for metastatic
             disease must be stopped at least 5 half-lives or 7 days, whichever is longer, before
             study inclusion.

               -  Patient is concurrently using other anti-cancer therapy.