Down syndrome is a genetic disorder caused by trisomy of chromosome 21. Trisomy 21 occurs in approximately 1 of every 1000 births. Chromosome 21 is an acrocentric chromosome, and trisomy 21 is the most common form of chromosomal trisomy. Trisomy 21 most often occurs as the result of nondisjunction of meiosis, which leads to an extra copy of chromosome 21. Some patients with Down syndrome have a Robertsonian translocation to chromosome 14 or chromosome 22, which are two other acrocentric chromosomes. In these cases, the number of total chromosomes is normal at 46, but the extra chromosome 21 material is translocated to another chromosome. This, in effect, causes an extra chromo- some 21. All or part of chromosome 21 may be trans-located, leading to variations in phenotype. Mosaicism is a rare cause of trisomy 21 in partial cell lines, and the clinical phenotype depends on how early the genetic defect occurred during embryogenesis.