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.