In 1970, Frye and Edidin published research describing the mobility of plasma membrane proteins. They fused mouse and human cells together, creating a hybrid cell, and then examined the localization of mouse and human proteins over time. Initially mouse and human proteins were each restricted to one-half of the heterokaryon, but over time the mouse and human proteins mixed, with each being present over the entire cell surface. What technique did Frye and Edidin likely use to examine the mouse and human proteins