Bromophenol blue in SDS-PAGE

Bromophenol blue is used as an indicator dye within the sample. Its main purpose is to show the progression of the proteins through the gel. It has a slight negative charge, making it migrate through the gel in the same direction as the proteins. It pools in front of the proteins, creating a dye front, which is the only visual cue to how far along the proteins have moved. The dye front helps the user know when to turn off the current before the samples run off the end of the gel. Additionally, the blue color of the sample makes it easier for the user to load the sample properly into the wells.

Gel showing one lane with blue lines indicating separated proteins and a blue line at the bottom of the gel, representing the dye front.

Figure 1. Dye front: Bromophenol Blue acts as a dye front on the gel.