Demonstration of the Philadelphia translocation in chronic myeloid leukemia (CML)

The Philadelphia translocation (1) in chronic
myeloid leukemia (CML, see p. 332) can be demonstrate
in metaphase (2) and in interphase (3)
by means of in situ hybridization. When a probe
for the BCR gene is used in interphase, the normal
signal consists of two fluorescing dots, one
dot on each chromosome 22. (On good preparations
of metaphase chromosomes, one dot is
seen over each chromatid and appears as a
double dot on a chromosome.) When the probe
includes the breakpoint of the translocation,
three signals are visible: the largest over the
normal chromosome 22, a small one over the
BCR sequences remaining in the distal long arm
of a chromosome 22 (22q), and another small
one over the sequences translocated to the distal
long arm of chromosome 9.