Confirmation of isochromosome 18p using whole chromosome arm-specific fluorescence in situ hybridization

Fluorescent in situ hybridization (FISH) using a pool of DNA clones that have been mapped to the short arm of chromosome 18 is described. This arm-specific painting technique was used to analyze the chromosomes of two patients who displayed different clinical pictures. One patient had clinical features consistent with tetrasomy 18p, and the other patient had some of the clinical features associated with trisomy 18. The initial karyotype of both patients showed the presence of a small metacentric marker chromosome, thought to resemble an iso(18p). This interpretation of the karyotype was confirmed for both cases using a specific chromosome arm-painting strategy. While whole-chromosome paints may identify the origin of markers or detect translocations, arm-specific libraries will permit detailed analysis of chromosomal rearrangements when more precise information may be needed.