To achieve a true, mirror-image protein with reciprocal stereospecific protein activity, PTMs must be incorporated into the D-polypeptide product. Serine and tyrosine phosphorylation are common PTMs. Whilst the L-serine equivalent (Table , Entry 34) is commercially available, the corresponding protected D-Ser building block must be accessed through chemical synthesis. This has been demonstrated in the synthesis of mirror-image ribosomal proteins which are essential in the binding of L-RNA molecules. Phosphorylation and sulfation of tyrosine residues have been incorporated into chemical synthesis of L-proteins using similar protected building blocks (Table , Entry 33 & 35). It is reasonable that equivalent building blocks can be prepared in D-enantiomeric form. Indeed, tyrosine phosphorylation is a PTM observed in many transcriptional regulators and has been shown to impact DNA binding. Glycosylation is another widespread PTM found in proteins. However, the sugars attached to the protein are chiral existing almost exclusively in D-form. Mirror-image glycoprotein would require installation of L-sugar polymers onto the D-polypeptide and has not yet been reported. However, quasi-racemic protein crystallography of synthetic glycoprotein Ser-CCL1 could be facilitated using the un-glycosylated D-enantiomer. In many cases, glycans can play important roles in protein or nucleotide binding, and therefore achieving mirror-image protein glycosylation is a key milestone in research surrounding Dproteins, particularly in the area of MIPD.