Direct alteration of a gene in the human genome requires an understanding of the role of the gene in metabolism. A gene may need to be introduced into a specific tissue or alternatively it may be possible to use accessible tissue such as bone marrow. The level of gene expression required also needs to be known as does the position in the genome into which the gene is to be inserted. Insertion of DNA needs to be of high efficiency and accuracy. Various methods are available including virus, the use of inert adjuvant, microinjection and electroporation. The procedure with the most potential for accuracy is the use of specially designed plasmids. The example of the use of such a plasmid in achieving target modification of the -globin gene is given. The method has high accuracy but low efficiency.