Isochromosome 17q: An abnormal chromosome 17 with two identical long (q) arms due to duplication of the long arm and loss of the short arm. Isochromosome 17q is the most common isochromosome in cancer. It plays an important role in tumor development and progression. Hematologic malignancies such as chronic myeloid leukemia (CML) with isochromosome 17q carry a poor prognosis. Isochromosome 17q is the most common chromosome abnormality in primitive neuroectodermal tumors and medulloblastoma. Isochromosome 17q is, by convention, symbolized as i(17q).
The breakpoint in chromosome 17 that permits the formation of isochromosome 17q is in band 17p11.2 within a region with large, palindromic, low-copy repeat sequences. This complex genomic architecture suggests that isochromosome 17q arises not as a random event but rather due to a susceptibility in the genomic structure.