Cell biologists studying the mobility of plasma membrane proteins fused mouse and human cells together, creating a heterokaryon. They then examined the localization of mouse and human proteins over time. Initially the mouse and human proteins were restricted to separate halves of the heterokaryon. Over time the mouse and human proteins mixed, with each protein eventually being present over the entire cell surface, consistent with proteins being able to move within the plane of the membrane. What techniques could have been used to examine the locations of the mouse and human membrane proteins