Following these experiments we focused on understanding Histidine selection, and more specifically on P1, as it was predicted to possess a narrower binding pocket. To do this, we solved the crystal structure of ParcuCDPS and explored the residues determining substrate selection. ParcuCDPS belongs to the XYP sub-group of the CDPS family, characterised by the presence of 3 residues: X40 where X is a non-conserved residue, Y202, and P203 (numbering respective to AlbC). Structures of three previous members from XYP are available , but these enzymes use relatively hydrophobic and non-polar amino acids such as glycine, alanine and leucine. We solved the crystal structure of ParcuCDPS at a resolution of 1.90 Å (9 residues out of the total 230 were not traceable), uncovering unique characteristics of histidine substrate selection (Fig. ). The structure was solved by Iodide SAD phasing after extensive failed trials of molecular replacement, suggesting significant deviation from previously determined CDPS structures.