The nuclear protein p100 is a proliferation-associated protein whose expression is restricted to cell cycle phases S, G2, and M. Exclusively expressed in proliferating cells from the transition G1/S until the end of cytokinesis. During mitosis it is strictly associated with the spindle pole and with the mitotic spindle, whereas during S and G2, it is diffusely distributed throughout the nucleus. The full-length cDNA encodes a 747-amino acid protein with a putative ATP/GTP-binding site motif. RT-PCR analysis demonstrated strong expression of in lung carcinoma cell lines but not in normal lung tissues. Expression was also found in adult placenta, skeletal muscle, thymus, testis, and small intestine and in fetal brain, liver, and kidney. P100 is also correlated to cancer prognosis.