▁critical ▁aspects ▁of ▁cell ular ▁bi ology . ▁For ▁example , ▁early ▁study ▁of ▁gene ▁expression ▁in ▁individual ▁cells ▁in ▁fruit ▁fly ▁emb ry os ▁allowed ▁scient ists ▁to ▁discover ▁regular ized ▁patterns ▁or ▁grad ients ▁of ▁specific ▁gene ▁trans cription ▁during ▁different ▁stages ▁of ▁growth , ▁allowing ▁for ▁a ▁more ▁detailed ▁understanding ▁of ▁development ▁at ▁the ▁level ▁of ▁location ▁and ▁time . ▁Another ▁phenomen on ▁in ▁gene ▁expression ▁which ▁could ▁only ▁be ▁identified ▁at ▁the ▁single ▁cell ▁level ▁is ▁oscill atory ▁gene ▁expression , ▁in ▁which ▁a ▁gene ▁is ▁expressed ▁on ▁and ▁off ▁period ically . ▁ ▁Single - cell ▁gene ▁expression ▁is ▁typically ▁ass ay ed ▁using ▁R NA - seq . ▁After ▁the ▁cell ▁has ▁been ▁isolated , ▁the ▁R NA - seq ▁protocol ▁typically ▁consists ▁of ▁three ▁steps : ▁ ▁the ▁R NA ▁is ▁reverse ▁trans cribed ▁into ▁c D NA , ▁the ▁c D NA ▁is ▁ampl ified ▁to ▁make ▁more ▁material ▁available ▁for ▁the ▁sequ encer , ▁and ▁the ▁c D NA ▁is ▁sequ enced . ▁ ▁Vari ation ▁in ▁DNA ▁sequence ▁ ▁A ▁population ▁of ▁single ▁cel led ▁organ isms ▁like ▁b acter ia ▁typically ▁vary ▁slightly ▁in ▁their ▁DNA ▁sequence ▁due ▁to ▁mut ations ▁acquired ▁during ▁reprodu ction . ▁Within ▁a ▁single ▁human , ▁individual ▁cells ▁typically ▁have ▁identical ▁genom es , ▁though ▁there ▁are ▁interesting ▁exceptions , ▁such ▁as ▁B - cell s , ▁which ▁have ▁variation ▁in ▁their ▁DNA ▁en abling ▁them ▁to ▁generate ▁different ▁ant ib od ies ▁to ▁bind ▁to ▁the