NA ▁apt am ers ▁have ▁high ▁stability ▁in ▁blood ▁ser um , ▁since ▁they ▁are ▁less ▁sus cept ible ▁to ▁be ▁cle aved ▁hydro ly t ically ▁by ▁en zym es . ▁They ▁are ▁ex cret ed ▁by ▁the ▁kid ne ys ▁in ▁a ▁short ▁time ▁due ▁to ▁their ▁low ▁mol ar ▁mass ▁( which ▁is ▁below ▁the ▁ren al ▁threshold ). ▁ ▁L - R NA ▁apt am ers ▁modified ▁with ▁a ▁higher ▁mol ar ▁mass , ▁such ▁as ▁P EG yl ated ▁L - R NA ▁apt am ers , ▁show ▁a ▁prolong ed ▁pl asma ▁half - life . ▁ ▁Production ▁Un like ▁other ▁apt am ers , ▁L - R NA ▁apt am ers ▁are ▁not ▁directly ▁made ▁using ▁system atic ▁evolution ▁of ▁lig ands ▁by ▁exponential ▁en rich ment ▁( SE LE X ), ▁as ▁L - n uc le ic ▁ac ids ▁are ▁not ▁am enable ▁to ▁en zym atic ▁methods , ▁such ▁as ▁polym er ase ▁chain ▁reaction ▁( PC R ), ▁used ▁in ▁SE LE X . ▁Therefore , ▁the ▁selection ▁is ▁done ▁with ▁mirror ed ▁target ▁mole cules . ▁ ▁Ref lection ▁of ▁the ▁target ▁mole c ule ▁The ▁first ▁step ▁is ▁the ▁production ▁of ▁the ▁target ' s ▁en anti omer . ▁In ▁the ▁case ▁of ▁pe pt ides ▁and ▁small ▁prote ins ▁that ▁are ▁produced ▁synth et ically , ▁an ▁en anti omer ▁is ▁made ▁using ▁synth etic ▁D - am ino ▁ac ids . ▁If ▁the ▁target ▁is ▁a ▁larger ▁protein