The present invention provides a drug containing a substance that inhibits a function of RET, such as an antibody against RET. In addition, the present invention provides a drug containing a substance that inhibits a function of GDNF, such as an antibody against GDNF. Moreover, the present invention provides a drug containing a substance that inhibits a function of GFRα1, such as an antibody against GFRα1. The drug of the present invention is useful as an agent for the prophylaxis or treatment of breast cancer and the like, an inhibitor of cancer cell growth and the like.
