B 4 8 ▁and ▁A po B 1 0 0 . ▁The ▁first ▁is ▁synth es ized ▁exclus ively ▁by ▁the ▁small ▁int est ine , ▁the ▁second ▁by ▁the ▁li ver . ▁A po B - 1 0 0 ▁is ▁the ▁largest ▁of ▁the ▁apo B ▁group ▁of ▁prote ins , ▁consisting ▁of ▁ 4 5 6 3 ▁am ino ▁ac ids . ▁Both ▁is o forms ▁are ▁c oded ▁by ▁AP OB ▁and ▁by ▁a ▁single ▁m R NA ▁trans cript ▁larger ▁than ▁ 1 6 ▁k b . ▁A po B 4 8 ▁is ▁generated ▁when ▁a ▁stop ▁cod on ▁( U AA ) ▁at ▁resid ue ▁ 2 1 5 3 ▁is ▁created ▁by ▁R NA ▁editing . ▁There ▁appears ▁to ▁be ▁a ▁trans - act ing ▁t issue - specific ▁sp lic ing ▁gene ▁that ▁determ ines ▁which ▁is o form ▁is ▁ultimately ▁produced . ▁Alternatively , ▁there ▁is ▁some ▁evidence ▁that ▁a ▁c is - act ing ▁element ▁several ▁thousand ▁b p ▁up stream ▁determ ines ▁which ▁is o form ▁is ▁produced . ▁ ▁As ▁a ▁result ▁of ▁the ▁R NA ▁editing , ▁A po B 4 8 ▁and ▁A po B 1 0 0 ▁share ▁a ▁common ▁N - term inal ▁sequence , ▁but ▁A po B 4 8 ▁la cks ▁A po B 1 0 0 ' s ▁C - term inal ▁L DL ▁re ceptor ▁binding ▁region . ▁In ▁fact , ▁A po B 4 8 ▁is ▁so - called ▁because ▁it ▁constit utes