You are interested in the mobility of two different unknown integral plasma membrane proteins—protein X and protein Y—in a cell. In one cell, you fluorescently label all the X proteins. In another cell, you label all the Y proteins. When you perform FRAP analysis on both cells, you find that most of the protein X fluorescence has recovered within 3 minutes, but recovery of the protein Y fluorescence is significantly slower. Even after 10 minutes, only 50% of the protein Y fluorescence has recovered.