To understand how BCAP31, a triple transmembrane protein mainly anchored in the ER (Figure 5A), functionally drives breast cancer development in cooperation with the membrane protein EGFR in a ligand-independent manner, we observed the intracellular distribution of BCAP31 and EGFR in the absence of ligand by confocal microscopy (Figure 5B). Here, BCAP31 is linked to breast cancer.