The development of molecular probes by using DNA sequences of differing sizes, complexity, and specificity, coupled with technological innovations such as multicolor fluorochromes, computerized signal amplification, and image analysis, makes fluorescent in situ hybridization (FISH) a powerful investigative tool for use in clinical cytogenetics (1 –3 ). FISH is rapidly becoming routine in the clinical laboratory repertoire and, in many cases, has replaced high-resolution cytogenetic analyses (for a comprehensive overview of the applications of FISH in the cytogenetics laboratory the reader may refer to Shaffer (1995) (4 ). Traditionally, routine cytogenetic analysis, with high-resolution banding levels of 650-850 bands per haploid karyotype, was limited to detecting deletions greater than 2-5 Mb in size. In contrast, by utilizing labeled DNA probes that are complementary to a desired gene or chromosomal locus, FISH analysis permits the detection of deletions significantly less than one Mb. In addition, FISH analysis has the distinct advantage of detecting not only cryptic deletions of a chromosomal locus but cryptic translocations (5 ) and as discussed below, cryptic duplications as well (4 ).