Two-dimensional SDS-PAGE Gel Electrophoresis:

Every amino acid has its own isoelectric point i.e., pI. pI is the pH at that the mobility of the amino acid is zero. Therefore, proteins that comprise amino acids do have a features pI value. In two-dimensional gel electrophoresis, proteins are separated according to their pI and then they are permitted to mix along with SDS. Denatured proteins are then permitted to enter a polyacrylamide gel.  The passage of the proteins through potential ultimately separates the proteins according to their masses.

Figure:  Spots found after staining in 2D gel electrophoresis