The prospects for gene therapy.

There are an estimated 100,000 genes in the human body. By replacing defective genes with healthy ones, it will be possible to treat many hereditary diseases such as haemophilia, cystic fibrosis and, eventually, some forms of muscular dystrophy and cancer.

Rapid progress has been made in recent years in characterizing hereditary diseases and in understanding their causes. These diseases are due to alterations of genes, which encode the information for building the body and making it work. Genes are the carriers of hereditary traits and are contained in filaments of DNA (deoxyribonucleic acid) that are present in the cells of all organisms. The precise number of genes in the human body is unknown, but it is estimated to be around 100,000.

Every cell has two copies of each gene, one deriving from the father, the other from the mother. The information in the genes is used for making proteins, the agents for building the body and making it work. A disease appears when a gene is altered in such a way that either it does not make its corresponding protein, or else makes a protein that is incapable of functioning. Since the same protein is encoded in both genes of each pair, if only one fails enough protein continues to be made to satisfy the needs of the organisms, and there is no disease. A disease appears when both fail and no functional protein is made. This generally happens when a child is born from two healthy parents, each of whom carries an altered gene of the same pair. Such parents have a one-in-four chance of generating a defective child.

Hereditary genetic diseases can vary greatly in severity. At one extreme are diseases such as phenylketonuria, which, after detection at birth, can be completely prevented by adopting a suitable diet. At the other are those like Lesch-Nyhan syndrome, which affects only boys, causing a terrible form of …