The spindle checkpoint, the primary mechanism to ensure that two daughter cells receive the same amount of DNA, is compromised in many malignant tumors and has been implicated as a contributor to aneuploidy and carcinogenesis. The extent of expression and subcellular localization of the spindle proteins Aurora A, Mad2, and BUBR1 varies considerably in different immunohistochemical (IHC) reports from archival tumor tissues. Given the conflicting reports in the literature about the localization of these proteins, we examined the subcellular localization of Aurora kinase A, Mad2, and BUBR1 in normal and cancerous human tissues by IHC. In normal tissues, Aurora A was mainly localized to the nucleus when monoclonal or purified polyclonal antibodies were used, and Mad2 was localized to the nucleus, whereas BUBR1 was localized to the cytoplasm. In malignant tissues, Aurora A showed additional staining in the cytoplasm in the majority of tumors analyzed. Furthermore, BUBR1 was also localized to both the nucleus and cytoplasm in a significant fraction of tumors. Subcellular localization of Mad2 was similar in normal and malignant tissues. Thus, the validity of some earlier IHC studies of Aurora A, Mad2, and BUBR1 should be reconsidered, indicating that high-quality antibodies and a high-alkaline antigen-retrieval technique are required to achieve optimal results. We conclude that the subcellular localizations of these spindle proteins are different, although they have overlapping biological functions, and that Aurora A and BUBR1 undergo a shift in the subcellular localization during malignant transformation.