Large clinically consequential imbalances detected at the breakpoints of apparently balanced and inherited chromosome rearrangements.

When a chromosome abnormality is identified in a child with a developmental delay and/or multiple congenital anomalies and the chromosome rearrangement appears balanced, follow-up studies often examine both parents for this rearrangement. If either clinically unaffected parent has a chromosome abnormality with a banding pattern identical to the affected child's study, then it is assumed that the chromosome rearrangement is balanced and directly inherited from the normal carrier parent. It is therefore unlikely that the chromosome rearrangement is responsible for the child's clinical presentation. We present two unrelated cases in which an identical and apparently balanced abnormal chromosome banding pattern was identified in both an affected child and an unaffected parent of that child. Despite the identical banding patterns, molecular characterization through genomic microarray and fluorescence in situ hybridization showed the parent to be balanced whereas the affected child was significantly unbalanced. These two cases emphasize the utility of genomic microarray for further characterization of apparently balanced inherited chromosome rearrangements and caution against the assumption that identical banding patterns between a child and parent represent identical genomic rearrangements.