Threonine / THR / T (EnzDes coloring)
Threonine (THR or T) is a hydrophilic amino acid. Its shape in Foldit is identical to that of valine. It's also similar in shape to asparagine or aspartate. The CPK or EnzDes coloring options (found in view options) can be used to help distinguish these amino acids.
In the Foldit Lua interface, the functions structure.GetAminoAcid and structure.SetAminoAcid use lowercase "t" for threonine.
For more, see Threonine on wikipedia.