We have developed a polymerase chain reaction (PCR) based technique to detect allelic loss. In this differential PCR a target gene and a reference gene are coamplified In the same reaction vessel. The ratio of the intensity of the two resultant bands is an Indication of relative gene dosage. This procedure is sensitive in that gene copy ratios of 2:1 and 3:2 (reference: target gene) can readily be detected. Using this differential PCR, we have examined 64 cases of chronic myelogenous leukemia (CML) for the loss of the β1-interferon gene, a relatively common event in certain human leukemias and lymphomas (1). Only one patient who was Philadelphia chromosome positive and who was in blast crisis exhibited allelic loss of the fl-interferon gene. Thus despite deletions at the β-interferon locus in the CML cell line, K562, this perturbation is rarely seen in primary CML samples.