AbstractThe ECT2 protooncogene plays a critical role in cytokinesis, and its C‐terminal half encodes a Dbl homology‐pleckstrin homology module, which catalyzes guanine nucleotide exchange on the Rho family of small GTPases. The N‐terminal half of ECT2 (ECT2‐N) contains domains related to the cell cycle regulator/checkpoint control proteins including human XRCC1, budding yeast CLB6, and fission yeast Cut5. The Cut5‐related domain consists of two BRCT repeats, which are widespread to repair/checkpoint control proteins. ECT2 is ubiquitously expressed in various tissues and cell lines, but elevated levels of ECT2 expression were found in various tumor cell lines and rapidly developing tissues in mouse embryos. Consistent with these findings, induction of ECT2 expression was observed upon stimulation by serum or various growth factors. In contrast to other oncogenes whose expression is induced early in G1, ECT2 expression was induced later, coinciding with the initiation of DNA synthesis. To test the role of the cell cycle regulator/checkpoint control protein‐related domains of ECT2 in cytokinesis, we expressed various ECT2 derivatives in U2OS cells, and analyzed their DNA content by flow cytometry. Expression of the N‐terminal half of ECT2, which lacks the catalytic domain, generated cells with more than 4N DNA content, suggesting that cytokinesis was inhibited in these cells. Interestingly, ECT2‐N lacking the nuclear localization signals inhibited cytokinesis more strongly than the derivatives containing these signals. Mutational analyses revealed that the XRCC1, CLB6, and BRCT domains in ECT2‐N are all essential for the cytokinesis inhibition by ECT2‐N. These results suggest that the XRCC1, CLB6, and BRCT domains of ECT2 play a critical role in regulating cytokinesis. Published 2003 Wiley‐Liss, Inc.