Well, because the proteins will have a net charge. And whenever they have a net charge, they will move within an electric field as a result of the interaction between the electric field and the charge, the net charge on that protein. So, for instance, if we take a protein that contains a net positive charge and we place it into our field, it will begin to move towards the negative end. And if we take a protein that contains a net negative charge, it will begin to move away from this end and towards this positively charged end. Now, the proteins will continue moving along our gel until they reach the PH value at which the overall charge is zero. And when the overall charge is zero, because there is no net charge, those proteins will no longer move along that electric field.