in contrast , substitution of val with either large aromatic amino acids ( that add bulk : phe , tyr ) or polar side chains ( ser , thr ) segregates ctl clones normally recognizing aa 278-286 into two groups , one that remains lytic ( permissive ) despite these changes and another that fails to lyse , indicating ctl can discriminate at a single aa .
C0301713
aromatic amino acids
C0031453
phe