Inclusion Criteria:

          -  For Part 1: Histologically- or cytologically-confirmed diagnosis of unresectable GIST
             or another advanced solid tumor. Patients with unresectable GIST must have disease
             that has progressed following imatinib and at least 1 of the following: sunitinib,
             regorafenib, sorafenib, dasatinib, pazopanib or an experimental kinase-inhibitor
             agent, or disease with a D842 mutation in the PDGFRα gene. Patients with an advanced
             solid tumor other than GIST must have relapsed or refractory disease without an
             available effective therapy.

        OR For Part 2:

          -  Group 1: Patients must have a confirmed diagnosis of unresectable GIST that has
             progressed following imatinib and at least 1 of the following: sunitinib, regorafenib,
             sorafenib, dasatinib, pazopanib, or an experimental kinase-inhibitor agent, and the
             patient does not have a D842V mutation in PDGFRα.

          -  Group 2: Patients must have a confirmed diagnosis of unresectable GIST with a D842V
             mutation in the PDGFRα gene. The PDGFRα mutation will be identified by local or
             central assessment, either in an archival tissue sample or a new tumor biopsy obtained
             prior to treatment with avapritinib.

          -  Group 3: Patients must have a confirmed diagnosis of unresectable GIST that has
             progressed and/or patients must have experienced intolerance to imatinib and not
             received additional kinase-inhibitor therapy. Patients must not have a known D842V
             mutation in PDGFRα.

          -  Groups 1, 2 and 3: At least 1 measurable lesion defined by mRECIST 1.1 for patients
             with GIST.

          -  Groups 1 and 2: A tumor sample (archival tissue or a new tumor biopsy) has been
             submitted for mutational testing.

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0-2

        Exclusion Criteria:

          -  QT interval corrected using Fridericia's formula (QTcF) >450 milliseconds

          -  Platelet count <90,000/mL

          -  Absolute neutrophil count <1000/mL

          -  Hemoglobin <9 g/dL

          -  Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) >3 x the upper
             limit of normal (ULN) if no hepatic metastases are present; >5 × ULN if hepatic
             metastases are present

          -  Total bilirubin >1.5 × ULN; >3 × ULN with direct bilirubin, >1.5 × ULN in the presence
             of Gilbert's Disease

          -  Estimated (Cockroft-Gault formula) or measured creatinine clearance <40 mL/min Brain
             malignancy or metastases to the brain

          -  History of a seizure disorder or requirement for anti-seizure medication

          -  Group 3: Patients known to be KIT wild type.