Huntington's chorea, formerly also referred to as St. Vitus' dance, is a severe hereditary neurodegenerative disease occurring in approximately four out of 100,000 persons. Ordinarily it does not become manifest until a person's middle years. It begins with uncontrolled movements, changing to variable moods, dementia and death. Six years ago, a group of 61 American and British researchers at eight universities discovered the gene responsible for the disease. It codes for an enormous protein of more than 3140 amino acid residues in a chain. In normal protein, this chain contains a series of up to 37 coupled glutamines. The only difference between the normal and the diseased proteins is the length of the glutamine series, which numbers fewer than 37 in healthy and more than 40 in diseased persons. The longer the glutamine series, the earlier the onset of the disease. By coincidence I found that long series of glutamines attach to each other like zippers, and I thought that this might supply the key to the molecular mechanism of Huntington's chorea. In my lecture, I will talk about the consequences of this mechanism in this and related hereditary diseases and their associations with Alzheimer's, Parkinsonism, and diseases caused by prions.