Approximate relationship between response, the putative number of leukemic cells, and the level of BCR-ABL transcripts. When a complete cytogenetic response has been achieved, the (putative) number of residual Ph⁺ cells can be measured only with quantitative molecular methods. The figure highlights the importance of molecular methods in the evaluation of the response to treatment. However, the sensitivity of current methods may vary substantially and in any case, no method can detect the transcript at very low cellular levels. For this reason the term “complete molecular response” may be misleading, since it might erroneously be interpreted as an equivalent of complete disease eradication and cure. The term “undetectable BCR-ABL” may better describe the biologic situation.