Diagnosis of tetrasomy 18p using in situ hybridization of a DNA probe to metaphase chromosomes.

We identified an isochromosome of 18p [47,XY, +i(18p)] conclusively by in situ hybridization of an 18p-specific probe (B74; D18S3) to metaphase chromosomes of an affected patient. Clinical findings included mental retardation, microcephaly, and an atrial septal defect. Although there is similarity to patients previously described with tetrasomy 18p, it is impossible to rule out a low frequency of misdiagnoses in karyotypes determined solely by standard cytogenetic analyses. We expect the ability to conclusively identify an i(18p) to lead to the delineation of tetrasomy 18p as a distinct clinical syndrome.