**3. Defining gene function**

One significant hurdle to the discussion of duplicate functional specialization is defining gene function. Gene function may be broken into two broad categories – regulation and gene product (MacCarthy & Bergman, 2007). Regulation encompasses the "when, where, why, and how much" aspects of a gene's transcription – non-coding elements around a gene (such as enhancers and signaling sequences) can direct when a gene should be expressed, and in what quantity. These non-coding elements are responsive to various cellular and environmental triggers. Changes to regulation alone may be sufficient to bring about the specialization of a new duplicate.

Gene products, on the other hand, primarily dictate the "how" in a gene's function (along with some regulatory and subcellular localization information present in the 5' and 3' untranslated regions (UTRs)). Studies of duplicated genes have focused on changes to various coding sequence properties, such as binding sites, eligible cofactors, indels, and catalytic residues (Turunen et al., 2009). It's theoretically possible for a duplicate gene to become functionally specialized without any change to its regulation (Des Marais & Rausher, 2008).

It is worth noting that changes falling into these two categories can occur serially or in concert. For example, a change in tissue localization may precede structural mutations adapting a protein to a new environment.
