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.

• Different degrees of developmental delay and intellectual disability
• Abnormal growth including low birth weight, failure to thrive, hypotonia (low muscle tone), and short stature
• Characteristic craniofacial features such as microcephaly (unusually small head); a sloping forehead with narrow temples; a broad nose with a bulbous tip and "slitlike" nostrils; a small jaw; abnormally wide fontanelles at birth; cleft lip and/or palate; low-set, misshapen ears; microphthalmia (unusually small eyes) and/or short, upwardly slanting eyelid folds (palpebral fissures)
• Vision problems
• Congenital heart defects
• Abnormalities of the muscles and/or bones such as congenital dislocation of the hips; abnormal position and/or limited function of the joints; underdevelopment of certain bones; and/or abnormal curvature of the spine
• Unusually formed feet, such as club foot or "rocker bottom" feet
• Abnormalities of the male reproductive system, including undescended testes, a small penis, and/or abnormal placement of the urinary opening
• Kidney problems
• Brain malformations such as hydrocephalus and/or Dandy-Walker malformation