FISH characterisation of dynamic mosaicism involving an inv dup(15) in a patient with mental retardation.

We report on a man with mental retardation and a complex karyotype with cells containing up to three morphologically distinct supernumerary marker chromosomes (SMCs) in most metaphases. Fluorescence in situ hybridisation studies using chromosome 15-specific probes characterised the presence of seven SMCs all derived from chromosome 15. The results suggest that the patient originally had a large inv dup(15) containing two copies of the Prader-Willi/Angelman critical region which became mitotically unstable, and by a process of dynamic mosaicism various morphologically distinct SMCs arose.