Expanded Access Program for Avapritinib

Inclusion Criteria: -Participant has received treatment with avapritinib in BLU-285-1107 clinical trial and continues to receive clinical benefit from avapritinib treatment, as assessed by the Investigator. Exclusion Criteria: * Participant requires ongoing treatment or has received treatment within 28 days before the start of avapritinib administration with drugs or foods that are strong CYP3A inhibitors or inducers. * Female participants of childbearing potential who are unwilling, if not postmenopausal or surgically sterile, to abstain from sexual intercourse or employ highly effective contraception from the time of informed consent and for at least 6 weeks after the last dose of study treatment. * Male participants who are unwilling, if not surgically sterile, to abstain from sexual intercourse or employ highly effective contraception from the time of informed consent and for at least 6 weeks after the last dose of study treatment. * Participant is pregnant, as documented by a serum β-hCG pregnancy test. Participants with β- hCG values that are within the range for pregnancy but are not pregnant (false-positives) may be enrolled with written consent of the Sponsor after pregnancy has been ruled out. * Female participants of nonchildbearing potential (premenarchal, bilateral tubal ligation, bilateral oophorectomy, or hysterectomy) do not require a serum β-hCG test. * Participant is breastfeeding. * Participants who require anticoagulants, with the exception of stable doses of prophylactic reversible anticoagulants. * Participants who are unable to swallow tablets.