ino ▁acid ▁that ▁replaced ▁it ▁after ▁the ▁mut ation . ▁Because ▁of ▁this , ▁the ▁matrix ▁ ▁is ▁assumed ▁to ▁be ▁symmetric , ▁and ▁the ▁entries ▁of ▁ ▁above ▁the ▁main ▁diagonal ▁are ▁computed ▁on ▁this ▁basis . ▁The ▁entries ▁along ▁the ▁diagonal ▁of ▁ ▁do ▁not ▁correspond ▁to ▁mut ations ▁and ▁can ▁be ▁left ▁un filled . ▁ ▁In ▁addition ▁to ▁these ▁counts , ▁data ▁on ▁the ▁mut ability ▁and ▁the ▁frequency ▁of ▁the ▁am ino ▁ac ids ▁was ▁obtained . ▁The ▁mut ability ▁of ▁an ▁am ino ▁acid ▁is ▁the ▁ratio ▁of ▁the ▁number ▁of ▁mut ations ▁it ▁is ▁involved ▁in ▁and ▁the ▁number ▁of ▁times ▁it ▁occurs ▁in ▁an ▁alignment . ▁Mut ability ▁measures ▁how ▁likely ▁an ▁am ino ▁acid ▁is ▁to ▁mut ate ▁accept ably . ▁As par agine , ▁an ▁am ino ▁acid ▁with ▁a ▁small ▁polar ▁side ▁chain , ▁was ▁found ▁to ▁be ▁the ▁most ▁mutable ▁of ▁the ▁am ino ▁ac ids . ▁ ▁C yst e ine ▁and ▁try pt oph an ▁were ▁found ▁to ▁be ▁the ▁least ▁mutable ▁am ino ▁ac ids . ▁The ▁side ▁ch ains ▁for ▁c yst e ine ▁and ▁try pt oph an ▁have ▁less ▁common ▁structures : ▁c yst e ine ' s ▁side ▁chain ▁contains ▁sul fur ▁which ▁particip ates ▁in ▁dis ulf ide ▁b onds ▁with ▁other ▁c yst e ine ▁mole cules , ▁and ▁try pt oph an ' s ▁side ▁chain ▁is ▁large ▁and ▁a rom atic . ▁Since ▁there ▁are ▁several ▁small ▁polar ▁am ino