There have been several reports of A domain residue positions that can be used in primary amino acid sequence alignments to predict substrate selectivity of new or uncharacterized A domains. The first report canonized the 10 amino acid code (10AA code or Stachelhaus code) of GrsA_A as a fingerprint for A domain substrate selectivity in bacteria, which was quickly followed by another group corroborating nine9 of the ten residues. The 10AA code has historically provided the best predictions for canonical bacterial A domains that activate L-α-amino acids. Despite its incorporation into many in silico prediction tools, it became clear that the 10AA code was not accurate in every case, leading to reports of expanded codes, many of which were determined from different A domain structures. A comparison of all codes that map onto GrsA_A numbering is shown in Table . Several of these expanded codes have been proposed based specifically on fungal A domain sequences, which helps to fill a significant gap in our current predictive abilities. Schwecke et al. first described siderophore synthetases in fungi using comparative modeling with the MODELLER program. They noted that the 10AA code was still present, but three additional residues were found to be essential for substrate binding, generating a 13AA code. Bushley and co-workers reported a 17AA code (17AA code A, Table ) based on evolutionary relationships of fungal NRPSs and not structural analysis. Upon crystallization of the first fungal A domain, which activated a bulky hydroxamate substrate, a second 17AA code was defined (17AA code B, Table ), as discussed in section 3.7. Despite multiple reports focused on fungal A domains, all of these studies have looked at siderophore pathways, which might not directly translate to other NRPS A domains.