AHAS is a thiamine diphosphate dependent enzyme containing a catalytic and regulatory structural subunits (CSU,RSU). The CSU portion has three domains, alpha (85-269), beta (281-458), gamma (463-639), and a C-terminal tail (646-668). It occurs in yeast (ScAHAS) or bacteria as a dimer and in others such as Arabidopsis Thaliana as a ~700 kDa tetramer composed of a dimer of dimers. In a single dimer there are 2 alpha-domains and 2 gamma-domains creating the central region and a beta-domain on either side. The tetramer form is adopted in the presence of herbicides and with or without valine its cryo-em structure is presented in [24, PDB 6VZ8, 6U9H] and 6U9H is shown in Figure . The base of each bulb is the RSU portion, and each bulb is made of a dimer (of dimers). There are 2 tunnels in each dimer (of dimers), one on either side and in the cleft near the base attachment. The x-ray structure of the CSU monomer unit in the presence of CS is given as PDB ID: 1YBH and without CS. It is in Figure . The active catalytic site appears accessible being on the surface of AHAS. However, in the naturally occurring dimer or tetramer there are 2 active sites each of which are located at the interface of the CSUs; these 2 catalytic sites are not surface accessible by solvent and are led to the outside through a tunnel (or surface channel). At the entrance of the channel is the herbicide binding site, containing P197 and other amino acids connected to susceptibility or resistance, such as W574S and resistant. The channel is shown in Figure ; several residues are highlighted in red and blue coming from 2 monomers in the dimer and the 4 important amino acids in green which are responsible for the catalytic activity.