BCR/ABL1 is the primary gene rearrangement associated with Chronic Myeloid Leukemia (CML), resulting from a translocation between chromosomes 9 and 22.

This rearrangement creates a protein that drives cellular growth out of control and leads to overproduction of certain blood cells.

FISH for BCR/ABL1 allows the translocation to be identified at diagnosis and monitored for disease progression or relapse.