Key Inclusion Criteria:

   - 1. Patient must have SM, confirmed by Central Pathology Review of BM biopsy, and
   central review of B- and C-findings by WHO diagnostic criteria.

   - 2. Patient must have moderate-to-severe symptoms based on minimum mean total symptom
   score (TSS) of the ISM Symptom Assessment Form (ISM-SAF) over the 14-day eligibility
   screening period.

   - 3. Patient must have failed to achieve adequate symptom control for 1 or more Baseline
   symptoms.

   - 4. For patients receiving corticosteroids, the dose must be ≤ 20 mg/d prednisone or
   equivalent, and the dose must be stable for ≥ 14 days.

   - 5. Patient must have an Eastern Cooperative Oncology Group (ECOG) performance status
   (PS) of 0 to 2.

Key Exclusion Criteria:

   - 1. Patient has been diagnosed with any of the following WHO SM subclassifications:
   cutaneous mastocytosis only, smoldering SM, SM with associated hematologic neoplasm,
   aggressive SM, mast cell leukemia, or mast cell sarcoma.

   - 2. Patient must not have received prior treatment with avapritinib.

   - 3. Patient must not have had any cytoreductive therapy including but not limited to
   masitinib and midostaurin, or investigational agent for < 14 days or 5 half-lives of
   the drug (whichever is longer), and for cladribine, interferon alpha, pegylated
   interferon, or antibody therapy < 28 days or 5 half-lives of the drug (whichever is
   longer), before beginning the 14-day ISM-SAF eligibility TSS assessment.

   - 4. Patient must not have received radiotherapy or psoralen and ultraviolet A (PUVA)
   therapy < 14 days before beginning the 14-day ISM-SAF eligibility TSS assessment.

   - 5. Patient must not have received any hematopoietic growth factor the preceding 14
   days before beginning the 14-day ISM-SAF eligibility TSS assessment.

   - 6. Patient must not have a QT interval corrected using Fridericia's formula (QTcF) of
   > 480 msec.