Scientists have generally thought that while humans have varied traits and physical appearances, the DNA sequence among individuals is 99.9% identical. Not so, according to a relatively new field of genetic study that has revealed that many individuals' chromosomes harbor deletions and duplications of large DNA segments varying in size from thousands to millions of DNA bases.
Such copy number variations (CNVs) of genetic material can lie within or around genes, leading to gene disruption or dosage imbalances, or outside of genes, which may or may not affect gene expression. Research is beginning to show that CNVs can be involved with disease occurrence and drug response and also can provide insight into human genome evolution.
Hampton T. Disease, Drug Response Linked to Loss or Gain of Big DNA Chunks in Genome. JAMA. 2007;297(14):1539-1540. doi:10.1001/jama.297.14.1539