When is it subfunctionalization and when is it not?

New genes arise from duplication followed by divergence. The most common fate of a duplication is one copy is deleted to return to a single copy. But if divergence occurs, the function of the gene can be split between the two copies, which is called subfunctionalization, or a new function can arise, which is called neofunctionalization. Changes in expression between the two copies does not necessarily suggest that subfunctionalization has occurred. In this commentary, the caveats and criteria for subfunctionalization are discussed.