Stands for internal protein - or internal protein, come to think of it. Internal to another protein, that is. One that is spliced out of the outer protein (the extein) to produce a mature host protein and a free intein.

This might seem a little bizarre, but that's biology for you. It is as if a 'virus-like' protein is embedded in a larger one. It is more precise to say that a protein does some autoprocessing on an internal, rather than a terminal, sequence. The obvious analogy is with introns in genes, although the intein gene is probably not an intron!

There is a database of inteins at:

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