The Ki-67 proteins were first discovered in an attempt to generate cancer specific monoclonal antibodies. A monoclonal antibody which bound to structures in the nuclei of dividing but not quiescent cells was produced and shown to bind two very large proteins of molecular weight 345kDa and 395kDa. The two proteins were derived from alternate transcripts of a single gene. The presence of Ki-67 proteins, detected with an appropriate antibody, is an indicator of cell proliferation and the level of Ki-67 expression is one of the most reliable biomarkers of proliferative status of cancer cells. The Ki-67 antibody was raised against a recombinant construct containing amino acids 1,111-1,490 of human Ki-67 isotype 1. The antibody can be used to identify dividing cells in rat and mouse brain and also works on paraffin sections of human tissues, where it is useful to identify cancer cells. Mouse select image at left for larger view.