Conclusions: We conclude that an important minority of patients are diagnosed with
atypical BCR–ABL1 transcripts, but their characterization is important for proper assessment
of treatment response to avoid false negative results of residual disease status. Based on the
individual RT-qPCR strategies, results cannot be expressed on the International Scale (IS) and
thus the common molecular milestones and guidelines for treatment discontinuation are
difficult to apply. We therefore suggest reporting individual molecular response (IMR) levels
in these cases as a time dependent log reduction of BCR-ABL1 transcript levels compared to
the BCR-ABL1 expression at baseline prior to therapy.