• Progression-free survival [ Time Frame: Median progression free survival (time when half of the patients have progressed) The measurements will be done bi-monthly. ] [ Designated as safety issue: No ]
  

• Time to tumor progression, response rate and overall survival;(every 2months [ Time Frame: TTP, RR OS: every 2 months ] [ Designated as safety issue: Yes ]
  
• Safety and tolerability of 400 mg bid nilotinib and 400 mg bid imatinib; [ Time Frame: Safety tolerability: all visits ] [ Designated as safety issue: Yes ]
  
• Mutational status of tumors with regard to Kit and PDGFR, correlating specific mutations with efficacy endpoints [ Time Frame: Mutational status- when available ] [ Designated as safety issue: Yes ]
  

Inclusion Criteria:

• Providing a written informed consent.
• Male or female patients ≥ 18 years of age at Visit 1;
• Histologically confirmed diagnosis of GIST of any anatomical location, which is unresectable and/ or metastatic prior to or at Visit 1;
• Positive immunohistochemical staining for c-Kit (CD117) or Kit negative,
• Documented disease progression according to RECIST on prior therapy with imatinib, 400 mg PO qd;

Exclusion Criteria:

• Prior use of imatinib doses higher than 400 mg qd or prior use of any other tyrosine-kinase inhibitor;
• Treatment with any cytotoxic and/or investigational cytotoxic drug less than or equal to 4 weeks (6 weeks for nitrosurea or mitomycin C) prior to Visit 1 with the exception of imatinib therapy.
• Concomitant antineoplastic treatments, such as chemotherapy, immunotherapy, biological response modifiers, or radiotherapy;
• Lesions that have been irradiated cannot be included as sites of measurable disease. If the only measurable lesion was previously irradiated, the patient cannot be included;
• Serious uncontrolled concomitant medical or psychiatric illness;