Genomes without genes

Human accessory chromosome (HAC) vectors, introduced in 1997 by Harrington et al. [225] demonstrated clones carrying up to 10 000 kb of DNA. These types of stable vectors are of more importance as gene transfer vectors to human cells to help annotate the human genome without disruption of the extant chromosomes and without use of potentially dangerous fragments of viral vectors. 

There is a normal variation of DNA sequence just as is true of more obvious aspects of human structute. Variations of DNA sequence, polymorphisms, occur approximately once in evety 500 nucleotides, or about 10 times per genome. There are without doubt deletions and insertions of DNA as well as single-base substitutions. In healthy people, these alterations obviously occur in noncoding regions of DNA or at sites that cause no change in function of the encoded protein. This heritable polymorphism of DNA structure can be associated with certain diseases within a large kindred and can be used to search for the specific gene involved, as is illustrated below. It can also be used in a variety of applications in forensic medicine. 

---