It's called the isoelectric point, or simply pi. So every protein contains this isoelectric point. Now, some proteins, if they consist of the same exact combination of ionizable amino acids, they will have the same isoelectric point. But usually proteins have different values, isoelectric point values, because they have different combinations of these ionizable amino acids. And because this is another property of proteins that is unique to most proteins, we can use this property to basically purify our protein. So if we have a mixture of different types of proteins, we can separate and isolate specific proteins from that mixture by using a method known as the isoelectric focusing method.