The DNA laboratory and neurological practice.

Molecular genetic techniques have already made an impact on neurological practice, leading to improved diagnosis and genetic counselling in inherited neurological disorders. The purpose of this review is to outline what the DNA laboratory can currently provide the neurologist, in terms of services to patients with neurological disease and their families. The first neurological disease to be assigned to a specific chromosomal region by means of random linkage analysis using variable DNA markers (polymorphisms) was Huntington's disease (HD).' Since then the neurological disease map has become much more dense2 (table 1). Also, several gene defects have been identified, either after gene mapping and subsequent gene isolation (reverse genetics, now known as positional cloning), such as, hereditary motor and sensory neuropathy (HMSN type Ia)34 or by direct analysis of candidate genes, for example, in some cases of familial Alzheimer's disease5 and inherited prion diseases6 (table 2). The small genome found in mitochondria has also been established as a mutation hot spot for some neurological diseases, a possibility suggested by their biochemical features and predominantly maternal mode of transmission.7