Detection of the migration of breast cancer cells in zebrafish embryos as an in vivo animal model. (a, b) RFP-labeled MDA-MB-231 cells were transfected with synthetic let-7a, CCR7 siRNA, and scRNA, respectively, and injected into the center of the yolk sac of transgenic zebrafish in which embryonic vessels are visualized with green fluorescence, as described in the Material and Methods section. After 1 day and 3 days of injections, RFP-labeled MDA-MB-231 cells were detected in GFP-labeled vessels by using a fluorescence microscope, and the results are presented as a photograph in (a). The number of migrating cells was counted (b). The data were derived from three replicated experiments. The scale bars are 200 μm and 50 μm in the last panels.