<P> Mutant Huntingtin is expressed throughout the body and associated with abnormalities in peripheral tissues that are directly caused by such expression outside the brain . These abnormalities include muscle atrophy, cardiac failure, impaired glucose tolerance, weight loss, osteoporosis, and testicular atrophy . </P> <P> All humans have two copies of the Huntingtin gene (HTT), which codes for the protein Huntingtin (HTT). The gene is also called HD and IT15, which stands for' interesting transcript 15' . Part of this gene is a repeated section called a trinucleotide repeat, which varies in length between individuals and may change length between generations . If the repeat is present in a healthy gene, a dynamic mutation may increase the repeat count and result in a defective gene . When the length of this repeated section reaches a certain threshold, it produces an altered form of the protein, called mutant Huntingtin protein (mHTT). The differing functions of these proteins are the cause of pathological changes which in turn cause the disease symptoms . The Huntington's disease mutation is genetically dominant and almost fully penetrant: mutation of either of a person's HTT alleles causes the disease . It is not inherited according to sex, but the length of the repeated section of the gene and hence its severity can be influenced by the sex of the affected parent . </P> <P> HD is one of several trinucleotide repeat disorders which are caused by the length of a repeated section of a gene exceeding a normal range . The HTT gene is located on the short arm of chromosome 4 at 4p16. 3 . HTT contains a sequence of three DNA bases--cytosine - adenine - guanine (CAG)--repeated multiple times (i.e....CAGCAGCAG ...), known as a trinucleotide repeat . CAG is the 3 - letter genetic code (codon) for the amino acid glutamine, so a series of them results in the production of a chain of glutamine known as a polyglutamine tract (or polyQ tract), and the repeated part of the gene, the PolyQ region . </P> <Table> Classification of the trinucleotide repeat, and resulting disease status, depends on the number of CAG repeats <Tr> <Th> Repeat count </Th> <Th> Classification </Th> <Th> Disease status </Th> <Th> Risk to offspring </Th> </Tr> <Tr> <Td> <26 </Td> <Td> Normal </Td> <Td> Will not be affected </Td> <Td> None </Td> </Tr> <Tr> <Td> 27--35 </Td> <Td> Intermediate </Td> <Td> Will not be affected </Td> <Td> Elevated but <<50% </Td> </Tr> <Tr> <Td> 36--39 </Td> <Td> Reduced Penetrance </Td> <Td> May or may not be affected </Td> <Td> 50% </Td> </Tr> <Tr> <Td> 40 + </Td> <Td> Full Penetrance </Td> <Td> Will be affected </Td> <Td> 50% </Td> </Tr> </Table>

What chromosome has the genetic mutation for huntington's disease