Although the AEC normalizes with effective therapy in MHES and can be used to monitor disease activity, data from chronic myelogenous leukemia and drug interruption trials in PDGFRA-associated HES suggest that molecular monitoring is preferable when possible since molecular relapse may precede hematologic (and clinical) relapse by several months (71). The gene discussed is PDGFRA; the disease is hypereosinophilic syndrome.