Avapritinib Rollover Study

Inclusion Criteria: * Participated in a Blueprint Medicines sponsored avapritinib clinical study. The Sponsor reserves the right to not roll over patients who have been on drug hold for an extended time period. * Completed End of Trial / End of Study (EOT/EOS) visit in the parent study and demonstrated compliance with the parent study requirements. * Continue to clinically benefit from treatment with avapritinib. * Able to give written informed consent. * Agree to continue to use highly effective contraception as defined in this protocol. * Female patients of childbearing potential must have negative highly sensitive serum pregnancy test within 20 days before the first dose of avapritinib. Women of nonchildbearing potential do not require this test. Exclusion Criteria: * Participant is participating in another interventional study. * Participant is unwilling or unable to comply with study procedures and study restrictions. * Participant is breastfeeding. Other protocol-defined criteria apply.