This next bit's pretty technical - don't say I didn't warn you!
Glivec is one of a new regime of drugs called signal transduction inhibitors (sorry you asked?) - hence the 'STI'. CML patients show a chromosomal oddity called Philadelphia chromosome-positive. ('Philadelphia' because it was first recognised in a lab near that city). Here a switch of genes takes place between chromosomes 9 and 22 which creates an abnormal protein called bcr-abl, a member of an enzyme family called tyrosine kinases which regulate cell growth. It is this which causes white blood cells to grow and multiply out of control. STI-571 shuts down this activity, specifically targeting cancer cells. This is its strength, it appears, in acting specifically on the aberrant cells rather than attack anything which grows, as in most chemotherapy.
If you would like to take this further, key in Glivec or Gleevec on a web search engine, where you will find a plethora of sites on the subject. It is fair to say that the initial euphoria has died down somewhat with further testing, but this drug is going to point the way to a whole new family of drugs.