Microtubule dynamics is an important factor in cell proliferation and one of the main targets of cancer chemotherapy. Since microtubule-associated proteins (MAPs) are known to influence microtubule stability, study of MAPs may contribute both to knowledge of cancer cell biology and to the production of new anti-cancer drugs. In this study, we identified a new mouse gene which is a homolog of human cytoskeleton-associated protein, CKAP2 gene, by differential display analysis. The level of expression of mouse CKAP2 (mCKAP2) was significantly higher in NIH3T3 cells expressing RET with a multiple endocrine neoplasia (MEN) 2A or MEN2B mutation than in parental NIH3T3 cells. Immunocytochemical analysis showed that mCKAP2 protein is localized in cytoplasm with a fibrillar appearance, and is co-localized with microtubules throughout the cell cycle. Furthermore, overexpression of mCKAP2 in cells appeared to stabilize microtubules against treatment with nocodazole, a microtubule-depolymerizing agent. In addition, levels of human CKAP2 were increased in some human tumor cell lines examined. These findings suggest that CKAP2 is a new MAP with microtubule-stabilizing properties and may represent a new molecular target for cancer chemotherapy.