At the initial stage of carcinogenesis, cell competition often occurs between 
newly emerging transformed cells and the neighboring normal cells, leading to 
the elimination of transformed cells from the epithelial layer. For instance, 
when RasV12-transformed cells are surrounded by normal cells, RasV12 cells are 
apically extruded from the epithelium. However, the underlying mechanisms of 
this tumor-suppressive process still remain enigmatic. We first show by electron 
microscopic analysis that characteristic finger-like membrane protrusions are 
projected from both normal and RasV12 cells at their interface. In addition, 
FBP17, a member of the F-BAR proteins, accumulates in RasV12 cells, as well as 
surrounding normal cells, which plays a positive role in the formation of 
finger-like protrusions and apical elimination of RasV12 cells. Furthermore, 
cdc42 acts upstream of these processes. These results suggest that the 
cdc42/FBP17 pathway is a crucial trigger of cell competition, inducing 
"protrusion to protrusion response" between normal and RasV12-transformed cells.