Down syndrome, or trisomy 21, occurs when an individual has three copies of chromosome 21 rather than two. Nondisjunction is the cause of trisomy 21 in 97% of cases and is a failure of the proper division of the chromosomes during cell division. Translocation accounts for the remaining 3% and is the exchange of parts between nonhomologous chromosomes during cell division. Chromosome breakage refers to a collection of rare syndromes in which a patient’s cells have elevated rates of chromosome rearrangements (i.e. Fanconi’s anemia) but is not associated with trisomy 21. Frameshift mutation refers to a type of gene mutation not a chromosomal abnormality.