Ki-67 antigen (KIA) is a nuclear protein that is expressed in proliferating cells, and thus can be used as a biomarker for proliferation. This protein is actively expressed during the G1, S, G2, and M phases of the cell cycle, while only a basal undetectable level of expression is seen in the G0 phase. Unlike the PCNA biomarker, KIA is not involved in any of the DNA repair processes, and is therefore, a more accurate indicator of the proliferative state of the cell. The percentage of Ki-67 positive cells in a tissue sample gives rise to the Ki67 index, which is used to estimate the number of growing cells in the neoplasm. Although the exact function of the protein is not known, it seems that it has some role in maintaining cellular proliferation.

Earlier Ki-67 antibodies could only be used in frozen or fresh tissue samples, and therefore caused serious difficulties in the assays. However, a monoclonal antibody against KIA, known as MIB-1 antibody, was later produced, which can be used with paraffin or formalin embedded tissues. KIA has since been used as a proliferation marker for both diagnostic and prognostic purposes in prostate cancer, benign menigiomas, non small cell lung carcinoma, ovarian cancer, and squamous cell carcinoma, and many other neoplastic growths.

The gene encoding KIA is located on the long arm of chromosome 10, where it spans a total length of around 30 kb, with its 14 coding exons. KIA consists of 3256 amino acids and weighs about 360 kDa. The protein is localized to the nucleus after expression. In the G1 phase, it is localized to the perinucleolar region, whereas later in the cell cycle, it can be detected throughout the nuclear matrix. During mitosis, KIA is detected on all the chromosomes.