Post-zygotic breakage of a dicentric chromosome results in mosaicism for a telocentric 9p marker chromosome in a boy with developmental delay

• Mosaic for the products of a dicentric chromosome breakage and a centric fission
• Formation of a telocentric marker chromosome due to centric fission
• Comparison of clinical features of 12 patients with chr9p inv dup del

Chromosomal rearrangements resulting in an inverted duplication and a terminal deletion (inv dup del) can occur due to three known mechanisms, two of them resulting in a normal copy region between the duplicated regions. These mechanisms involve the formation of a dicentric chromosome, which undergo breakage during cell division resulting in cells with either an inverted duplication and deletion or a terminal deletion. We describe a mosaic 3 year old patient with two cell lines carrying a chromosome 9p deletion where one of the cell lines contains an additional telocentric marker chromosome. Our patient is mosaic for the product of a double breakage of a dicentric chromosome including a centric fission. Mosaicism involving different rearrangements of the same chromosome is rare and suggests an early mitotic breakage event.Chr9p terminal deletions associated with duplications have previously been reported in 11 patients. We compare the clinical features of all 12 patients including the patient that we report here. To the best to our knowledge this is a first case reported where the double breakage occurred in the dicentric derivative chromosome 9.