AC S ▁of ▁positive ▁cell ▁selection ▁( des ired ▁group ). ▁ ▁Fl u ores cent ▁D yes ▁in ▁cell ▁sorting ▁ ▁Fl u ores cent ▁d yes ▁can ▁act ▁very ▁differently . ▁Gener ally , ▁a ▁flu ores cent ▁d ye ▁will ▁be ▁excited ▁by ▁a ▁coh er ent ▁light ▁source ▁( a ▁las er ) ▁at ▁a ▁particular ▁w avel ength ▁and ▁emit ▁light ▁at ▁a ▁lower ▁energy ▁and ▁longer ▁w avel ength . ▁The ▁most ▁common ▁d yes ▁act ▁by ▁binding ▁to ▁ant ig ens ▁presented ▁on ▁cells . ▁Common ▁ant ig ens ▁target ed ▁are ▁clusters ▁of ▁different iation ▁( CD s ). ▁These ▁are ▁specific ▁to ▁a ▁certain ▁type ▁of ▁cell . ▁If ▁you ▁can ▁identify ▁which ▁CD ▁is ▁presented ▁on ▁your ▁cells ▁of ▁interest , ▁then ▁you ▁can ▁st ain ▁your ▁sample ▁with ▁a ▁flu ores cent ▁d ye ▁specific ▁to ▁it ▁and ▁using ▁F AC S , ▁sort ▁out ▁only ▁those ▁cells . ▁However , ▁there ▁are ▁many ▁other ▁mechan isms ▁by ▁which ▁flu ores cent ▁d yes ▁can ▁act . ▁ ▁Some ▁d yes ▁are ▁able ▁to ▁diff use ▁across ▁memb ran es . ▁By ▁taking ▁advantage ▁of ▁this ▁property ▁of ▁the ▁d ye , ▁users ▁can ▁character ize ▁int rac ell ular ▁activity ▁as ▁well ▁as ▁surface - expression ▁of ▁prote ins . ▁For ▁example , ▁in ▁dead ▁cells , ▁prop id ium ▁i od ide ▁( PI ) ▁can ▁pen et rate ▁the ▁nucle us ▁where ▁it ▁bind s ▁to ▁DNA . ▁The