2-5% of patients with Angelman syndrome have imprinting defects. Deletions of the imprinting center (IC) region occur in 10-40% of patients with an IC defect and occur more frequently in familial cases. Epigenetic defects of the IC region are thought to comprise the remaining patients in this category and occur in sporadic cases. Patients in this group have a phenotype similar to those in the UPD group.
Up to 3% of patients with Prader-Willi syndrome have a microdeletion of the imprinting center.