A patient with isochromosome 18q, radial-thumb aplasia, thrombocytopenia, and an unbalanced 10;18 chromosome translocation.

We report on the clinical and cytogenetic findings in a newborn with a de novo isochromosome 18q. Radial/thumb aplasia and thrombocytopenia were significant features in addition to multiple congenital anomalies. Comparison with reported cases suggests that the genes for such features are located on the 18q arm. An additional finding of a non-reciprocal translocation between chromosome 18p telomere and chromosome 10q telomere was also observed in a majority of cells examined. This additional rearrangement likely has minimal phenotypic consequences, but does raise the possibility that cryptic translocations of telomeric ends of the deleted arm in isochromosome cases may be more common than appreciated.