A chromosomal translocation resulting in a fusion gene. The chromosomal translocation that fuses the ABL gene with the BCR gene occurs when a piece of chromosome 9 (with ABL) is exchanged with a piece of chromosome 22. The BCR-ABL fusion gene causes leukemia. The small chromosome resulting from the translocation (the chromosome 22 derivative), which carries the fusion gene, is referred to as the Philadelphia chromosome, after the city where the translocation was first identified and studied.