['Activating mutations in the RET receptor tyrosine kinase are responsible for the development of medullary thyroid cancer (MTC) in persons with Multiple Endocrine Neoplasia type 2. We hypothesized that STI571 (Gleevec) would inhibit RET kinase and be a useful agent in the treatment of MTC.', 'We determined the IC50 of STI571 for RET using an in vitro kinase assay and also examined the effects of STI571 on cellular proliferation and viability in TT cells, a human MTC cell line.', 'The average in vitro IC50 of STI571 for RET is 37 microM +/- 4 microM. Additionally, TT cells incubated with 10 microM STI571 for up to 8 days showed no apparent reduction in cell proliferation or viability. Higher concentrations of STI571, from 25 to 100 microM, induced necrosis of TT cells.'] Are rET tyrosine kinase and medullary thyroid cells unaffected by clinical doses of STI571?