Pallister–Killian syndrome (PKS) is a multisystem sporadic genetic condition characterized by facial anomalies, variable developmental delay and intellectual impairment, hypotonia, hearing loss, seizures, pigmentary skin differences, temporal alopecia, diaphragmatic hernia, congenital heart defects, and other systemic abnormalities. PKS is typically caused by the presence of a supernumerary isochromosome composed of the short arms of chromosome 12 resulting in tetrasomy 12p, which is often present in a tissue limited mosaic state. The PKS phenotype has also often been observed in individuals with complete or partial duplications of 12p (trisomy 12p rather than tetrasomy 12p) as the result of an interstitial duplication or unbalanced translocation. We have identified a proposita with PKS who has two small de novo interstitial duplications of 12p which, along with a review of previously reported cases, has allowed us to define a minimum critical region for PKS. © 2012 Wiley Periodicals, Inc.