So histidine is very commonly found in the active sites of many different enzymes, as we'll see in our study of biochemistry. So we have Lysine and Arginine, which are always positively charged on their side chain groups at the physiological PH of seven, histidine, which is also basic, like Lysine, Arginine. But the thing about HistoGene is it can either be positively charged or neutral at the normal physiological PH. Now, let's move on to the two acidic amino acids, aspartate and glutamate. So what do we mean by acidic amino acids? Well, acidic simply means that the side chain groups will have a full negative charge at the normal physiological PH, and that's because the PKA value of the side chains will be relatively low, will be below seven.