Immuno-electron microscopy of MCF-7 breast cancer cells demonstrates that Rab27B is localized on the membrane surface of MVE, but also smaller secretory granule-like structures (Figure 1), which suggests that in cancer cells Rab27 small GTPases are not restricted to the regulation of MVE secretion, but also other secretory vesicle types. This evidence concerns the gene RAB27A and breast cancer.