Certain malignancies have characteristic chromosome abnormalities. These can be chromosome deletions (the whole chromosome is absent or only a portion of a chromosome); additions (e.g., trisomy, when a third chromosome is present in a group that normally would consist of two); translocation, either single (where part of one chromosome breaks off and attaches to another) or reciprocal (where two chromosomes exchange a portion of each chromosome); or gene rearrangement (on the same chromosome). The most famous chromosome abnormality is the Philadelphia chromosome of chronic myelogenous leukemia (CML), present by standard chromosome analysis in about 85% of cases and by nucleic acid probe for gene rearrangement in about 95%–97% of cases (also present in about 25% of adult ALL, 5% of childhood ALL, and 1%–2% of adult acute myelogenous leukemia [AML]. This is a reciprocal translocation in which a portion of the long arm of chromosome 22 breaks off at an area known as the breakpoint cluster region (BCR or ph1 oncogene) and attaches to the long arm of chromosome 9, while the distal portion of the long arm of chromosome 9 (known as the abl oncogene) breaks off and replaces the missing part of chromosome 22. Chromosome 22 becomes shorter but finishes with part of the BCR oncogene still in place plus the addition of the ab1 oncogene, creating a very abnormal chromosome. In genetic terminology, the various changes involved in the Philadelphia chromosome abnormality are summarized by t(9;22) (q34;q11); t is the translocation; (9;22) are the chromosomes involved, with the lowest chromosome number placed first; (q34;q11) is the location of the changes on the chromosomes (q = long arm of the chromosome, p = short arm; the first number refers to a region; the second is a band within the region [chromosome quinocrine banding method]; a decimal followed by a number is a subband). Other symbols are del (deletion), inv (inversion), qh+ or – (long arm increased or shortened), ph+ or – (short arm increased or shortened), I (isochromosome [mirror-image chromosome composed of 2 long arms or 2 short arms]).