In many cases, the first two letters of the
   genetic code are more critical in specifying an amino acid.

   For  example,  the  codon for valine (val) only depends on the first 2
   letters (GU).

   The  3rd  position  of  the codon can "wobble": a single tRNA can pair
   with more than one codon in mRNA.

   U  at the 1st position (5') of the tRNA anticodon may pair with A or G
   at  the  3rd  position (3') of the mRNA codon, and G may likewise pair
   with U or C.

   Inosine (I), a modified base found in tRNA, may pair with C, U, or A.