A main feature of Huntington's disease (HD) is a loss of neurones from a region of the brain known as the striatum. A study of brain tissue taken from people with HD showed higher than normal (increased) urea concentrations in the anterior striatum and cerebellum regions of the brain. Urea is a toxic waste product of amino acid and protein metabolism. It is produced from ammonia, which is also toxic, in an enzyme-controlled cycle known as the urea cycle. Scientists carried out a study using transgenic (genetically modified) sheep, known as OVT73, to investigate the increased urea concentrations. OVT73 have the mutant allele of the Huntingtin (HTT) gene. At the time of the study, OVT73 were 5 years old and had no detectable loss of striatal neurones or disease symptoms. Samples of tissue were taken from the same region of the anterior striatum in: • six OVT73 (three females and three males) • a control group of non-transgenic sheep.