An isochromosome is an unbalanced structural abnormality in which the arms of the chromosome are mirror images of each other. The chromosome consists of two copies of either the long (q) arm or the short (p) arm because isochromosome formation is equivalent to a simultaneous duplication and deletion of genetic material. Consequently, there is partial trisomy of the genes present in the isochromosome and partial monosomy of the genes in the lost arm. Nomenclature An isochromosome can be abbreviated as i(chromosome number)(centromeric breakpoint). For example, an isochromosome of chromosome 17 containing two q arms can be identified as i(17)(q10).(Medulloblastoma) Mechanism Isochromosomes can be created during mitosis and meiosis through a misdivision of the centromere or U-type strand exchange. Centromere misdivision Under normal separation of sister chromatids in anaphase, the centromere will divide longitudinally, or parallel to the long axis of the chromosome. An isochromosome is created when the centromere is divided transversely, or perpendicular to the long axis of the chromosome. The division is usually not occurring in the centromere itself, but in an area surrounding the centromere, also known as a pericentric region. It is proposed that these sites of exchange contain homologous sequences between sister chromatids. Although the resulting chromosome may appear monocentric with only one centromere, it is isodicentric with two centromeres very close to each other; resulting in a potential loss of genetic material found on the other arms. Misdivision of the centromere can also produce monocentric isochromosomes, but they are not as common as dicentric isochromosomes. U-type strand exchange A more common mechanism in the formation of isochromosomes is through the breakage and fusion of sister chromatids, most likely occurring in early anaphase of mitosis or meiosis. A double-stranded break in the pericentric region of the chromosome is repaired when the sister chromatids, each containing a centromere, are fused together. This U-type exchange of genetic material creates an isodicentric chromosome. Misdivision of the centromere and U-type exchange can occur in sister chromatids, thus creating an isochromosome with genetically identical arms. However, U-type exchange can also occur for homologous chromosomes which creates an isochromosome with homologous arms. This exchange between homologues is most likely due to homologous sequences containing low copy repeats. Regardless of the chromosome involved in U-type exchange, the acentric fragment of the chromosome is lost, thus creating a partial monosomy of genes located in that portion of the acentric chromosome. Consequences The most