While obtaining homogenous recombinantly prepared posttranslationally modified (PTM'd) proteins remains a major technical challenge, chemical protein synthesis offers exquisite atomistic control and thus ensures homogeneity (see also Section 3). Racemic protein crystallography of PTM'd proteins was first applied to the glycosylated chemokine Ser-CCL1 protein, for which no structure was reported. The protein was synthesized in the native L-form, followed by site-specific, asparagine Nglycosylation with the native biantennary D-glycan. Synthesis of the corresponding D-enantiomer without glycosylation enabled the co-crystallization of the quasi-racemic protein (Figure ).