The Development of a Highly Specific Monoclonal Antibody against Ki‐67 Useful for Immunohistochemistry

Ki‐67 is a nuclear protein that is a useful marker of proliferation. Ki‐67 antibodies are used to generate a Ki‐67 labeling index, which is the percentage of cells with nuclear immunostaining in immunohistochemistry. The Ki‐67 labeling index may be useful as a prognostic and predictive marker in cancer. The Ki‐67 labeling index relies on the use of antibodies with a suitable level of sensitivity and specificity, and many Ki‐67 antibodies have been developed over the last two decades. One of the most widely used Ki‐67 antibodies is the mouse monoclonal MIB1. However, there have been many reports showing that in some cases MIB1 detects an unexpected membranous and cytoplasmic staining pattern in immunohistochemistry. Since Ki‐67 is exclusively expressed in the nucleus this raises questions about the specificity of MIB1. We have created a nearly comprehensive microarray of ~17,000 human proteins to investigate the specificity of MIB1 and identify cross reacting proteins. The protein microarray was also used to screen new panels of mouse monoclonals against Ki‐67. We have identified the antibody UMAB107 that in immunohistochemistry performs with similar sensitivity as MIB1, but unlike MIB1, UMAB107 is highly specific.