Mosaic trisomy 9 is a chromosomal abnormality that can affect may parts of the body. In people affected by this condition, some of the body's cells have three copies of chromosome 9 (trisomy), while other cells have the usual two copies of this chromosome. The signs and symptoms vary but may include mild to severe intellectual disability, developmental delay, growth problems (both before and after birth), congenital heart defects, and/or abnormalities of the craniofacial (skull and face) region. Most cases are not inherited; it often occurs sporadically as a random event during the formation of the reproductive cells (egg and sperm) or as the fertilized egg divides. Treatment is based on the signs and symptoms present in each person. The disease is mosaic trisomy 9 (MONDO_0020490, a Monarch Disease Ontology term). Also known as: Mosaic trisomy type 9.