Cancer metastasis is the most deadly stage in cancer progression.Despite significant efforts over the past decades,it remains elusive why only a very small fraction of cancer cells is able to generate micrometastasis and metastatic colonization.Here we show that when injected into the yolk of a 2 day-post-fertilization(dpf)embryo of Tg(fli1:EGFP or kdrl:mCherry)zebrafish,tumor-repopulating cells(TRCs),a highly tumorigenic subpopulation of mouse melanoma cells,are much more efficient in surviving and growing at various secondary sites to generate micrometastasis and colonization than control melanoma cells that were grown on rigid plastic.The metastasis of TRCs is dependent on the presence of Sox2,a self-renewal gene,and silencing Sox2 leads to the inhibition of TRC metastasis.High-resolution of 3D confocal images of the TRCs at the secondary sites(tail)show that extravasation and formation of micrometastases by TRCs is more efficient than by the control cells,and that this process was due to cdc42 downregulation mediated elevation of cell deformability in order for them to effectively squeeze through the vessels and the ECM imparted by Sox2-mediated cdc42 inhibition,as demonstrated by in vivo and in vitro transmigration assays.Our findings suggest that it is possible to intervene the extravasation dynamics during metastasis through inhibition of tumor cell deformability.