Instability of isochromosome 4p in a child with pure trisomy 4p syndrome features and entire 4q-arm translocation.

Constitutional chromosome instability so far has mainly been associated with ring formation. In addition, isochromosome formation involving the short arm with translocation of the entire long arm is rarely observed. This type of rearrangement has been reported for chromosomes 4, 5, 7, 9, 10, 12, and 20. Here, we present the third patient having an isochromosome 4p with 4q translocation, but showing for the first time chromosome instability detected by FISH following chromosome microarray analysis.