The present invention provides a novel antitumor agent with a novel action mechanism. The novel antitumor agent comprises, as the active ingredient, a substance capable of inhibiting a cell cycle-dependent Rho GTPase activating protein (RhoGAP). The RhoGAP, which depends on the cell cycle, plays an important role in the course of the acquisition of metastatic ability and/or invasive ability by cancer cells. By targeting the RhoGAP, the invasion and/or metastasis of cancer cells can be controlled. As the substance capable of inhibiting the cell cycle-dependent RhoGAP, an antisense oligonucleotide against an RhoGAP-encoding gene, an oligonucleotide inducing RNA interference, etc. can be cited. An oligonucleotide containing a synthetic nucleic acid such as BNA is preferred because of the high stability thereof. The present invention also provides a screening method for selecting an antitumor agent which is capable of suppressing the invasion and/or metastasis of cancer cells by a novel mechanism. This method for screening a novel antitumor agent is characterized in that a substance capable of inhibiting a cell cycle-dependent Rho GTPase activating protein is selected thereby.La présente invention concerne un nouvel agent antitumoral avec un nouveau mécanisme daction. Le nouvel agent antitumoral comprend, en tant que principe actif, une substance capable dinhiber une protéine activatrice Rho GTPase dépendant du cycle cellulaire (RhoGAP). La RhoGAP, qui dépend du cycle cellulaire, joue un rôle important au cours de lacquisition de capacité métastatique et/ou de capacité invasive par les cellules cancéreuses. En ciblant la RhoGAP, linvasion et/ou la métastase des cellules cancéreuses peut être contrôlée. Un oligonucléotide antisens contre un gène codant de RhoGAP, un oligonucléotide induisant linterférence par ARN, etc, constituent autant de substances capables dinhiber la RhoGAP dépendant du cycle cellulaire. Un oligonucléotide contenant un acide nucléique synthétique co
