Protein isoform

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.

The discovery of isoforms could explain the small number of protein coding regions genes revealed by the human genome project: different proteins encoded by the same gene could increase the diversity of the proteome. Isoforms at the DNA level are readily characterized by cDNA transcript studies. Many human genes possess confirmed alternative splicing isoforms. It has been estimated that ~100,000 ESTs can be identified in humans. Isoforms at the protein level can manifest in deletion of whole domains or shorter loops, usually located on the surface of the protein.