A
protein isoform is any of several different forms of the same
protein. Different forms of a protein may be produced from
very closely related
gene duplicates—as 'same protein' or 'a protein' makes no sense for highly diverged
paralogs that arose from a single ancestral gene that duplicated billions of years ago and subsequently diverged greatly in sequence, structure and functionality—or may arise from the same gene by
alternative splicing. In older literature one can also encounter the use of the term isoform to describe
alleles of the same gene, but currently the terms refers mostly to paralogous and alternatively spliced transcripts, not alleles.