Even monozygotic twins, who develop from one zygote, have infrequent genetic differences due to mutations occurring during development and gene copy number variation.

Gene copy number can be elevated in cancer cells.

It has been suggested that increased gene copy number might allow for the mass production of proteins required to meet the metabolic demands of embryogenesis and early development.

This means that genes promoting costly altruistic behaviour in their carriers can sometimes spread because they promote the survival of gene copies in relatives.

Gene set pre-processing selects the longest protein-coding transcript of alternatively spliced genes and of very similar gene copies.