A version or variant of a protein, which may come from different copies of a gene (e.g. homologs, typically paralogs) or from different processing and/or usage of a single gene (e.g., alternative promoter usage, alternative adenylation site usage, alternative splicing, alternative translational start sites). Different isoforms of a protein might be expressed (i.e., made) in different intracellular compartments (e.g., mitochondria vs. cytoplasm) and/or in different cell types and/or tissues (e.g. liver vs. heart vs. muscle vs. brain) and/or at different times in a cell’s life. Having such different forms allows for separate regulation and unique properties best suited for their needed roles.
