.

             The HGP's continued emphasis is on obtaining a complete and highly accurate reference sequence (1 error in 10,000 bases) that is largely continuous across each human chromosome. Scientists believe that knowing this sequence is critically important for understanding human biology and for applications to other fields. .

             The next step to gene therapy is to find a vehicle to transport the new genes. This is done through the use of vectors. A vector is a DNA molecule into which a DNA fragment can be cloned and which can replicate in a suitable host organism. The majority of vectors used today are attenuated or modified viruses. "The modified viruses can not replicate in the patient but do retain the ability to efficiently deliver genetic material."9 There are two main modes used to deliver the genes of interest to the patient: non-viral and viral delivery vectors have been used. Both of them have advantages, but also a not-so-short list of disadvantages. .

             "Non-viral vectors represent basically the approach of direct injection of the genetic material (the DNA) into the tissue. This is very straight forward, methodically not very difficult and has proven good results in animal experiments for a transient (non-stable) expression of these genes. However, it is difficult to precisely locate the area of action and the efficiency is not very good. To increase efficiency, the DNA is added to substances (e.g. polylysine) that allow the genes to cross cell membranes. But still, the expression is transient and injections have to be performed repeatedly to maintain a constant expression level.

             Viral vectors can be split in different groups: retroviral vectors, lentiviral vectors, and adenoviral vectors. A few more are under investigation. The principle of all these vectors is the packaging of the DNA into the viral capsule by replacing part (or all) of the viral genes. By removing the crucial viral genes, the viruses also loose their pathogenic effect.