With more than 4000 literature citations, the MIB-1 antibody has now been established as an important monoclonal mouse antibody for the demonstration of the Ki-67 antigen in formalin-fixed, paraffin-embedded specimens. The Ki-67 antigen is a large nuclear protein (345, 395 kDa) preferentially expressed during all active phases of the cell cycle (G1
, S, G2
and M-phases), but absent in resting cells (G0
-phase). The antibody is a useful aid for classification of a variety of tumors.