▁based ▁on ▁the ▁atomic ▁numbers ▁of ▁atoms ▁near ▁the ▁asym metric ▁carbon , ▁c yst e ine ▁( and ▁s elen oc yst e ine ) ▁have ▁R ▁ch ir ality , ▁because ▁of ▁the ▁presence ▁of ▁sul fur ▁( or ▁selenium ) ▁as ▁a ▁second ▁neighbour ▁to ▁the ▁asym metric ▁carbon . ▁ ▁The ▁remaining ▁ch iral ▁am ino ▁ac ids , ▁having ▁l ighter ▁atoms ▁in ▁that ▁position , ▁have ▁S ▁ch ir ality . ▁ ▁Diet ary ▁sources ▁ ▁Like ▁other ▁common ▁am ino ▁ac ids , ▁c yst e ine ▁( and ▁its ▁ox id ized ▁dim er ic ▁form ▁c yst ine ) ▁is ▁found ▁in ▁high - prote in ▁food s . ▁Although ▁class ified ▁as ▁a ▁non ess ential ▁am ino ▁acid , ▁in ▁rare ▁cases , ▁c yst e ine ▁may ▁be ▁essential ▁for ▁inf ants , ▁the ▁elder ly , ▁and ▁individuals ▁with ▁certain ▁met abol ic ▁dise ases ▁or ▁who ▁suffer ▁from ▁mal abs or ption ▁synd rom es . ▁C yst e ine ▁can ▁usually ▁be ▁synth es ized ▁by ▁the ▁human ▁body ▁under ▁normal ▁phys i ological ▁conditions ▁if ▁a ▁sufficient ▁quantity ▁of ▁m eth ion ine ▁is ▁available . ▁▁▁ ▁Like ▁other ▁am ino ▁ac ids , ▁in ▁its ▁mon omer ic ▁" free " ▁form ▁( not ▁as ▁part ▁of ▁a ▁protein ) ▁c yst e ine ▁has ▁an ▁am ph oter ic ▁character . ▁ ▁Indust rial ▁sources ▁▁ ▁The ▁majority ▁of ▁- c yst e ine ▁is