As can be seen in Figure , additional electron density between cysteines at position 114, facing each other in the crystal contact, is present in the HheG D114C structure. The shape and size of this additional electron density fit to that of the cross-linker BMOE, confirming effective cross-linking at the desired position. This extra density can be found for every cysteine 114 within the asymmetric unit, suggesting a high degree of cross-linking with BMOE for HheG D114C CLECs. Interestingly, there is no previous literature precedent, where cross-links within a biocatalytic CLEC could be visualized. This is explained by the fact that previously glutaraldehyde has been used almost exclusively as cross-linker for CLEC generation. As mentioned before, glutaraldehyde is able to react with different amino acids within a protein and is further prone to polymerization. This results in a multitude of cross-linking possibilities that will be occupied only randomly within a CLEC, hampering visualization by X-ray crystallography. In contrast, our approach yields highly defined cross-linking sites for stable CLEC generation and, thus, permits also visualization of resulting cross-links.