Other fusion-related mechanisms were observed. An example is shown in Fig. (Movie S1), where a GUV "gives birth" to a smaller GUV from its interior lumen. The interior GUV adheres to the membrane of the outer GUV before translocating to the exterior of this vesicle within a few seconds. We speculate that the internal GUV becomes hemifused with the outer membrane to facilitate this translocation event. It should also be noted that the outer GUV is initially non-spherical and gradually loses its large excess membrane area to become near-spherical during the time course of our observation window (~2 min).