They are all caused by additional repeats of the DNA sequence 'CAG' in a gene. This causes additional copies of the amino-acid glutamine to be inserted into the

They are all caused by additional repeats of the DNA sequence 'CAG' in a gene. This causes additional copies of the amino-acid glutamine to be inserted into the corresponding protein. Consequently, Bad Things happen (eg. the protein molecules aggregate into insoluble complexes), resulting in disease. The specific protein that is affected depends on the disease (huntingtin, ataxin-1, ataxin-2, ataxin-3, ATN1, and the androgen receptor for Huntington's, SCA1, SCA2, SCA3/MJD, DRPLA, Kennedy's Disease, and SBMA, respectively). The best reference for genetic diseases is the Online Mendelian Inheritance in Man database: http://omim.org/ http://omim.org/