[Tools of molecular biology, what can be expected from them?]

The techniques of molecular biology represent a new tool for research and diagnosis. They have been used routinely since the middle 80s. The first applications were mainly in hereditary diseases. These techniques investigated an information and not direct biological activity and so new strategies had to be developed. Two techniques have played key roles. They are Southern's method which enables the detection of specific sequences among DNA fragments in any individual within a few days, and the method of in vitro selective amplification (PCR) which is the equivalent of cloning a sequence of several hundred pairs of bases in any individual in less than 3 hours. In hereditary diseases, molecular biological techniques enable diagnosis of the genetic abnormality responsible for the condition, even when the defect is not known. To this end, two strategies are available. The first, and the most satisfactory, is the direct strategy of characterising the genetic defect itself. This is possible when the mutation is a major alteration of the DNA molecule such as a deletion, an insertion or a recombination or when the mutation is isolated and known. In other cases, an indirect approach may be used which consists of determining whether the subject has received normal or defective chromosomes from his or her parents. The identification of good or bad chromosomes is based on the study of DNA markers: polymorphism. The indirect strategy can only be used under certain conditions: presence of an index case, informativity of polymorphisms, complete family study.(ABSTRACT TRUNCATED AT 250 WORDS)