The
Philadelphia chromosome or
Philadelphia translocation is a specific abnormality of
chromosome 22, which is unusually short, as an acquired abnormality that is most commonly associated with
chronic myelogenous leukemia (CML). It is the result of a reciprocal
translocation between chromosome 9 and chromosome 22, which is specifically designated t(9;22)(q34;q11). This gives rise to a
fusion gene,
BCR-ABL1, that juxtaposes the
ABL1 gene on chromosome 9 (region q34) to a part of the
BCR ("breakpoint cluster region") gene on
chromosome 22 (region q11). The presence of this translocation is a highly
sensitive test for CML, since 95% of people with CML have this abnormality (the remainder have either a cryptic translocation that is invisible on
G-banded chromosome preparations, or a variant translocation involving another chromosome or chromosomes as well as the long arm of chromosomes 9 and 22). However, the presence of the Philadelphia (Ph) chromosome is not sufficiently
specific to diagnose CML, since it is also found in
acute lymphoblastic leukemia (ALL, 25–30% in adult and 2–10% in
pediatric cases) and occasionally in
acute myelogenous leukemia (AML).