Cytogenetic studies of Ph-positive leukemic patients and their parents have indicated that chromosome 22 involved in the formation of the t(9;22) is of maternal origin, whereas chromosome 9 is preferentially of paternal origin. These data have suggested that the two genes BCR and ABL, which become fused through the translocation, might be imprinted, ie expressed in a parental-specific manner. Recent molecular genetic studies however, have shown that BCR and ABL are expressed on both alleles and that the maternal and paternal ABL genes contribute equally often to the BCR-ABL fusion messenger. The findings make imprinting of these genes unlikely as an explanatory model and necessitate a combined cytogenetic and molecular genetic study.