What it is
E920 (L-cysteine hydrochloride) is an amino acid used as a flour treatment agent and dough conditioner. The cheapest commercial source is hydrolysed duck or chicken feathers; older or non-EU production sometimes uses human hair (mostly sourced from Chinese barber shops) or animal hides.
Why it matters
Common in commercial bread, pizza dough, pastries, and bagels — anywhere a manufacturer wants softer, more uniform dough. The label may simply say "L-cysteine" or list it only as E920.
What to do
Treat as not vegan unless the brand explicitly confirms a microbial or synthetic source (some specialty bakers use fermentation-derived L-cysteine and state so). Subway in the UK historically used feather-derived E920; many supermarket own-brand breads still do.