▁atom . ▁ ▁This ▁comp ound ▁is ▁sold ▁as ▁a ▁di et ary ▁supp lement , ▁and ▁used ▁as ▁an ▁ant id ote ▁in ▁cases ▁of ▁ac et amin oph en ▁over d ose . ▁ ▁She ep ▁C yst e ine ▁is ▁required ▁by ▁sheep ▁to ▁produce ▁w ool . ▁It ▁is ▁an ▁essential ▁am ino ▁acid ▁that ▁must ▁be ▁taken ▁in ▁from ▁their ▁feed . ▁ ▁As ▁a ▁consequence , ▁during ▁dr ought ▁conditions , ▁sheep ▁produce ▁less ▁w ool ; ▁however , ▁trans gen ic ▁sheep ▁that ▁can ▁make ▁their ▁own ▁c yst e ine ▁have ▁been ▁developed . ▁ ▁Diet ary ▁restrictions ▁The ▁animal - origin ating ▁sources ▁of ▁- c yst e ine ▁as ▁a ▁food ▁add itive ▁are ▁a ▁point ▁of ▁cont ention ▁for ▁people ▁following ▁di et ary ▁restrictions ▁such ▁as ▁k os her , ▁hal al , ▁veg an ▁or ▁veget arian . ▁To ▁avoid ▁this ▁problem , ▁- c yst e ine ▁can ▁also ▁be ▁s our ced ▁from ▁micro b ial ▁or ▁other ▁synth etic ▁processes . ▁ ▁See ▁also ▁▁ ▁A min o ▁ac ids ▁ ▁C yst e ine ▁met abol ism ▁ ▁C yst in uria ▁ ▁S ull ivan ▁reaction ▁ ▁References ▁ ▁External ▁links ▁ ▁C yst e ine ▁MS ▁Spect rum ▁ ▁International ▁Kid ney ▁Stone ▁Institute ▁ ▁http :// www . chem ie . fu - ber lin . de / chem istry / bio / am ino ac id / c yst ein ▁en .