As previously discussed, prognostic features for ALL include age, initial white count, ploidy, and associated translocation. However, The most important prognostic feature is disease response to cytotoxic treatment, which has traditionally been measured by morphological examination of the peripheral blood after 1 week of induction therapy or the bone marrow after 2 weeks of induction therapy. Those with persistent peripheral blast or bone marrow examination with 1% to 4% after Day 15 induction were found to have a poor prognosis . Although the morphological examination found that those with persistent disease have a poor prognosis, many with residual leukemia were not detected. The morphological examination determines a child to be in remission when less than 5% blasts are present in the bone marrow sample, which equates to 1010 undetectable leukemia cells or 1% residual leukemia . The significance of the morphological examination is now being replaced by the detection of minimal residual disease (MRD) through flow cytometry and/or polymerase chain reaction (PCR) , which has a 100-fold increase in sensitivity over the traditional examination. The technique of MRD though flow cytometry and/or PCR detects residual leukemia though the abnormal immunophenotype, amplification of an antigen receptorgene, or amplification of fusion transcripts.