There is a type of disease-causing gene that has in its nucleotide sequence (see genetic code) a variable number of tandem repetitions of a particular set of three nucleotides (trinucleotides). In these cases the number of trinucleotide repeats determines the severity of the disease. As an example myotonal dystrophy (MyD) is an autosomal dominant disease that involves progressive muscular degeneration but is very variable in its expression in different people. The trinucleotide involved in MyD is CTG (cytosine, thymine, guanine), which in the gene of unaffected individuals is found to be repeated about ten times. In moderately affected persons, who have difficulty in relaxing contracted muscles, there are about 100 repeats of this trinucleotide in the gene. In those who are severely affectedÑwith an expectation of death by age 25 because of drastic muscle degenerationÑthe gene contains over 1,000 repeats of CTG. There are many affected family-lines that are characterized by a slightly affected grandparent, one or more moderately affected offspring in the next generation (parent, aunt, uncle), and one or more severely affected grandchildren. It is not known what causes the progressive amplification of the repeated trinucleotide set from one generation to the next. Other genetic diseases where the manifestation, age of onset, and degree of severity of the disease are controlled by gene amplification include Huntington disease, spinocerebellar ataxia, and fragile X syndrome.