the duplication of a gene or region of a chromosome. This results in multiple copies of a gene and through different mutations and selection pressures, can lead to two different genes. The second copy of the gene or region does not necessarliy undergo the same selection pressure as the original gene, so mutations can occur at a faster rate in the copy, and can be maintained for longer periods of time. Gene duplication can cause copy number variations (CNV), or differences in the number of copies of a particular genetic sequence. CNVs have been linked to the cause of some diseases, such as cancer and schizophrenia.