Philadelphia chromosome
The Philadelphia chromosome is a famous abnormal chromosome formed when chromosomes 9 and 22 swap pieces, leaving an unusually short chromosome 22. It was the first chromosomal change ever firmly tied to a specific cancer, and it remains a textbook example of how a single rearrangement can switch a cell toward malignancy.
The swap is a reciprocal translocation written t(9;22). It joins part of the BCR gene on chromosome 22 to part of the ABL1 gene from chromosome 9, creating a fused BCR-ABL1 gene. This hybrid gene encodes a constantly active enzyme (a tyrosine kinase) that keeps telling the cell to grow and divide, driving chronic myeloid leukemia.
Its story is also a triumph of targeted medicine. Because the disease depends on the BCR-ABL1 enzyme, drugs designed to block that specific protein can control the leukemia with far less harm than older chemotherapy. The Philadelphia chromosome thus illustrates the full arc from cytogenetic discovery to molecular mechanism to precision treatment.
Detecting BCR-ABL1 by FISH or PCR confirms the Philadelphia chromosome and guides the choice of a targeted tyrosine-kinase inhibitor.
One translocation, one fusion enzyme, one targeted drug — a model of precision oncology.
The Philadelphia chromosome is a somatic (acquired) change found in the leukemia cells, not an inherited mutation passed to children. It is named after the city where it was discovered in 1960.