Inclusion Criteria:

• Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information (PHI) in accordance with national and local subject privacy regulations.
• Age greater than or equal to 18 years at the time of informed consent.
• Pathologically confirmed GIST refractory to, intolerant of, or not a candidate for imatinib and sunitinib therapy.
• FDG PET standard uptake value (SUVmax; averaged over a maximum of 5 lesions) greater than or equal to 2 at screening.
• ECOG performance status of less than or equal to 2.
• Lab values consistent with adequate renal hepatic and bone marrow function.
• Must utilize effective contraception.

Exclusion Criteria:

• Prior treatment with imatinib, sunitinib, or sorafenib with in 14 days of day 1.
• Prior treatment with Hsp90 inhibitors at any time.
• Prior antitumor therapies including prior experimental agents, approved antitumor small molecules (excluding imatinib, sunitinib, or sorafenib) and biologics, or radiotherapy with in 28 days or <3 half lives (whichever is longer) before start of BIIB021treatment.
• Diabetes treated with insulin and/ or concurrent severe or uncontrolled other medical disease (i.e, systemic infection, hypertension, coronary artery disease, congestive heart failure).
• History of/ or predisposition to seizures.