Inclusion Criteria:

- Must be at least 18 years of age at the time of study entry

- Must have histologically confirmed metastatic and/or unresectable GIST

- Must have measurable disease on CT or MRI as defined by RECIST

- Must have documented failure (due to either progression or intolerance)of at least
prior imatinib and sunitinib. Previous administration of other known heat shock
protein 90 (Hsp90) inhibitors is permitted

- Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1

- Must have acceptable laboratory values as defined in the protocol

Exclusion Criteria:

- Known CNS metastases

- Major surgery within 4 weeks prior to receiving STA-9090

- Use of any investigational agents within 2 weeks or 6 half-lives of the agent,
whichever is shorter prior to receiving STA-9090

- No treatment with chronic immunosuppressants

- Must have otherwise adequate health status as defined in the protocol

- Left ventricular ejection fraction (LVEF) < than or = 50% at baseline

- Baseline QTc > 470 msec

- Pregnant or lactating females