What it is
E627 is disodium 5'-guanylate. Commercial production today is by fermenting plant sugars with bacteria; historical production used sardines or dried mushrooms.
Why it matters
Flavour enhancer often paired with E621 MSG. The half of E635 that is unambiguously vegan.
What to do
Vegan in modern industrial production. The closely-related E626 (guanylic acid) and E628 (dipotassium guanylate) are likewise vegan today.