5. BCR-ABL-positive, chronic phase CML patients with a transcript level according to the international scale (IS) of at least MR4, or better (MR4.5, MR5). MR4 is defined as (i) detectable disease ≤0.01% BCR-ABL IS or (ii) undetectable disease in cDNA with ≥10,000 ABL or ≥24,000 GUS transcripts for at least one year. There have to be at least three documented PCR-results with MR4 or better within the last year (+/-2 months) before study entry. One of these PCR´s must be a confirmatory MR4 measurement prior to randomization by an EUTOS-certified laboratory at the Universities of Mannheim or Jena. No PCR-results in the last year before randomisation can be worse than MR4