PCR profiles of patients with CEL following imatinib therapy. Dose of imatinib for each patient is shown on the left. Analysis was restricted to patients with sufficiently high levels of FIP1L1-PDGFRA expression to allow detection of MRD at a sensitivity of at least 1 in 1000. Patients treated and maintained on imatinib achieved molecular complete remission, whereas discontinuation of imatinib in 2 patients (UPN 3, UPN 17) was associated with rapid disease progression. UPN indicates unique patient number; alt, alternate day dosing schedule; IM, imatinib.