I was intrigued by a recent article in the journal Chem (link) entitled "Creation of Bacterial Cells with 5-hydroxytryptophan as a 21st Amino Acid Building Block" by Chen et al. in the group of Han Xiao at Rice University, wherein they make a strain that metabolically produces 5-hydroxytryptophan for genetic code expansion. It is an interesting example of why metabolic engineering is non-trivial and how scientific research does not progress in a logical fashion.