2D Electrophoresis, biotinylation and avidin blotting identifies surface-labeled oolemmal proteins. 2D electrophoresis of 800 zona-free mouse egg proteins, surface labeled with 2 mg/ml Sulfo-NHS biotin, and separated by 2D electrophoresis. Blotted with streptavidin-HRP and visualized by ECL (A). Corresponding 2D electrophoretic gel of 800 unlabeled zona-free mouse eggs visualized by silver staining. Labelled black arrows indicate previously identified protein spots (B). Horizontal axis of each shows isoelectric point and vertical axis shows molecular weight (kDa).