 Proteins can be separated on the basis of size by using an SDS-PAGE gel, or on the basis of size and their electric charge by using what is known as a 2D gel electrophoresis.The terms "northern", "western" and "eastern" blotting are derived from what initially was a molecular biology joke that played on the term "Southern blotting", after the technique described by Edwin Southern for the hybridisation of blotted DNA.Patricia Thomas, developer of the RNA blot which then became known as the "northern blot", actually didn't use the term.