IEF is an electrophoretic method that separates proteins according to their isoelectric points (pI). Proteins are amphoteric molecules; they carry either positive, negative, or zero net charge, depending on the pH of their surroundings (see Figure 7). The net charge of a protein is the sum of all the negative and positive charges of its amino acid side chains and amino- and carboxyl- termini. The isoelectric point is the specific pH at which the net charge of the protein is zero. Proteins are positively charged at pH values below their pI and negatively charged at pH values above their pI. If the net charge of a protein is plotted versus the pH of its environment (see Figure 7), the resulting curve intersects the abscissa at the isoelectric point.