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.

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.