What is Ponceau S?

Ponceau S is a stain commonly used for visualizing proteins that have been separated by electrophoresis, particularly in techniques such as Western blotting. When proteins are transferred to a membrane after electrophoresis, Ponceau S can be applied to stain the proteins, allowing for qualitative analysis of the transfer efficiency and the presence of specific proteins. The staining provides a way to confirm that the proteins are properly immobilized on the membrane before any further analysis, such as immunodetection, is carried out.

This use of Ponceau S is particularly advantageous because it is a non-permanent stain that can be easily washed away, which is useful for subsequent steps in the analysis. The other options refer to different concepts in the field of electrophoresis but do not correctly describe what Ponceau S specifically is.