Kinetics of molecular response to imatinib therapy. The rate of decline of FIP1L1-PDGFRA fusion transcripts normalized to the ABL gene following start of imatinib therapy is shown. PCR-positive results are depicted as colored data points, with log reduction as compared with pretreatment level marked on Y-axis on left. PCR-negative results are depicted as white data points, with the sensitivity for each respective PCR negative sample shown on the right-hand axis. In patient UPN 3, discontinuation of imatinib was followed by a rise in fusion transcript level associated with molecular relapse; a second molecular remission was obtained following reinstitution of imatinib. UPN indicates unique patient number.