Partial duplication 4q and deletion 1p36 in monozygotic twins with discordant phenotypes

We report on monozygotic (MZ) twins with a de novo chromosome abnormality consisting of a partial duplication of chromosome 4 (q25-qter) and deletion of chromosome 1p36. These infants had dysmorphic facial features and other clinical manifestations similar to those described with the previously delineated duplication 4q and deletion 1p36 phenotypes and two other reported cases of combined partial duplication 4q and deletion 1p36. However, the twins were discordant for a number of congenital anomalies. The discordant phenotypes described in these genetically identical infants demonstrate that nongenetic factors may play a significant role in the phenotypic differences in patients with recognized chromosome duplication and deletion syndromes, which are usually attributed to the individual genotypic differences in the duplicated and/or deleted chromosome segments.