Perhaps more than any other tumor marker associated with hematological neoplasia, the PMLIRAR-α hybrid gene has been shown to be important in the chinical practice. This aberration is absolutely APL-specific, being found in virtually 100% of cases and in no other tumors. Secondly, it identifies a clinical entity that is unique in its response to a specific treatment, i.e., the differentiative agent all-trans retinoic acid (ATRA). As the disease frequently presents with a life-threatening hemorrhagic diathesis, its prompt recognition in order to start the specific treatment is mandatory (1 , 2 ). In this respect, reverse-transcription polymerase chain reaction (RT-PCR) amphficatron of the specific fusion gene represents an extremely useful diagnostic tool. Finally, several groups have independently reported that RT-PCR monitoring studies of residual disease in APL provide important prognostic informations, by predicting hematologic relapse in patients who test positive during clinical remission (3 –5 ).