And what that means is histidine can exist in his protonated or deprotonated state at a neutral PH. And this really depends on what the local environment is and what the local conditions are around this amino acid. Now, this group here is known as the immediazel group, and it has a PKA of 6.0. Now, which one of these atoms will gain that H atom and be protonated? Well, it's this nitrogen here. So at the physiological PH, this is the nitrogen that can be protonated.