by the protein's structure/function, as well as the cell type and developmental stage during which they are produced. Determining specificity becomes more complicated when a protein has multiple subunits and each subunit has multiple isoforms.

For example, the 5' AMP-activated protein kinase (AMPK), an enzyme, which performs different roles in human cells, has 3 subunits:
* ?, catalytic domain, has two isoforms: ?1 and ?2 which are encoded from PRKAA1