Organs with macroscopically observed abnormalities were removed and fixed with 20% neutral buffered formalin solution (Wako Pure Chemical Industries, Osaka, Japan). Tissue specimens were also prepared under the CIEA guidelines. Each organ was cleaved according to the CIEA tissue assessment guidelines for rasH2 model mouse, and one hematoxylin and eosin stained tissue specimen was prepared per organ. Similarly, according to the provisions of CIEA, histological evaluation was performed with an optical microscope BX-50F (Olympus, Tokyo, Japan) to assess the presence of tumors. Moreover, the presence or absence of CNTs was confirmed with optical, fluorescence, and polarization microscopy using the same tissue sample.