The immense efforts involved in positional cloning of a disease gene are rarely rewarded with as much insight into the biology of the condition as has been the case with the dementias. Indeed, stages subsequent to cloning are often even more arduous. Table d and T.ab!e,,,,3 give a list of disorders with a behavioural phenotype (excluding metabolic conditions), due either to abnormalities of a single gene or of a small chromosomal region, analysed at the molecular level. For some conditions, mutant genes have been identified; for others, the cause remains obscure despite immense amounts of genetic analysis. Often, even where a gene has been found, its function is unknown.
Table 2 Cloned genes responsible for mental retardation
We can interpret the tables in two ways: either they suggest that molecular approaches are not an efficient way of understanding the pathogenesis of genetic conditions, or the pathogenesis is extremely complex. Unfortunately the latter may well be the case. A description of what is known about the relevant conditions is given below.
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