Key Inclusion Criteria

          -  Measurable, histologically/cytologically confirmed metastatic colorectal cancer
             (mCRC).

          -  Able to provide a sufficient amount of representative tumor specimen for central
             laboratory testing of RAS mutation status and microsatellite stable (MSS).

               -  If a fresh tissue sample is provided, a blood sample is required.

          -  Metastatic colorectal cancer (mCRC) categorized as microsatellite stable (MSS) by
             polymerase chain reaction (PCR) per local assay at any time prior to Screening or by
             the central laboratory.

          -  RAS mutation per local assay at any time prior to Screening or by the central
             laboratory.

          -  Have received at least 1 prior line of therapy and meets at least one of the following
             criteria:

               -  were unable to tolerate the prior first-line regimen

               -  experienced disease progression during or after prior first-line regimen for
                  metastatic disease

               -  progressed during or within 3 months of completing adjuvant chemotherapy. Note:
                  Generally, treatments that are separated by an event of progression are
                  considered different regimens.

          -  Have received no more than 2 prior lines of therapy (maintenance therapy given in the
             metastatic setting will not be considered a separate regimen). Generally, treatments
             that are separated by an event of progression are considered different regimens.

          -  Adequate bone marrow, cardiac, kidney and liver function

          -  Able to take oral medications

          -  Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1.

          -  Female patients are either postmenopausal for at least 1 year, are surgically sterile
             for at least 6 weeks, or must agree to take appropriate precautions to avoid pregnancy
             from screening through follow-up if of child-bearing potential

          -  Non-sterile male patients who are sexually active with female partners of childbearing
             potential must agree to follow instructions for acceptable or highly effective
             method(s) of contraception for the duration of study treatment and for 7 months after
             the last dose of study treatment with nivolumab

        Key Exclusion Criteria

          -  Prior treatment with any MEK inhibitor, an anti-PD-1, anti-PD-L1, anti-PD-L2,
             anti-CD137, or anti-CTLA-4 antibody, or any other antibody or drug specifically
             targeting T-cell co-stimulation or checkpoint pathways.

          -  Any untreated central nervous system (CNS) lesion.

          -  Patients with an active, known or suspected autoimmune disease. Patients with type I
             diabetes mellitus, hypothyroidism only requiring hormone replacement, skin disorders
             (such as vitiligo, psoriasis, or alopecia) not requiring systemic treatment, or
             conditions not expected to recur in the absence of an external trigger are permitted
             to enroll.

          -  Known history of retinal vein occlusion (RVO).

          -  Known history of Gilbert's syndrome.

          -  Pregnant or breastfeeding females.

          -  Treatment with systemic immunosuppressive medications (including but not limited to
             prednisone, cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor
             necrosis factor [anti-TNF] agents) within 2 weeks prior to first day of study
             treatment:

          -  History of thromboembolic or cerebrovascular events ≤ 6 months prior to starting study
             treatment, including transient ischemic attacks, cerebrovascular accidents, deep vein
             thrombosis or pulmonary emboli.

          -  Uncontrolled hypertension defined as persistent systolic blood pressure ≥ 150 mmHg or
             diastolic blood pressure ≥ 100 mmHg despite current therapy.

          -  Concurrent neuromuscular disorder that is associated with the potential of elevated
             creatine kinase (CK) (e.g., inflammatory myopathies, muscular dystrophy, amyotrophic
             lateral sclerosis, spinal muscular atrophy).

          -  History or current evidence of retinal vein occlusion (RVO) or current risk factors
             for RVO (e.g., uncontrolled glaucoma or ocular hypertension, history of hyperviscosity
             or hypercoagulability syndromes).

          -  Known history of positive test for human immunodeficiency virus (HIV) or known
             acquired immunodeficiency syndrome (AIDS). NOTE: Testing for HIV must be performed at
             sites where mandated locally.

          -  Any positive test for hepatitis B virus or hepatitis C virus indicating acute or
             chronic infection, and/or detectable virus.