In eukaryotes, the activation of the prereplicative complex and assembly of an active DNA unwinding complex are critical but poorly understood steps required for the initiation of DNA replication. In this report, we have used bimolecular fluorescence complementation assays in HeLa cells to examine the interactions between Cdc45, Mcm2-7, and the GINS complex (collectively called the CMG complex), which seem to play a key role in the formation and progression of replication forks. Interactions between the CMG components were observed only after the G1/S transition of the cell cycle and were abolished by treatment of cells with either a CDK inhibitor or siRNA against the Cdc7 kinase. Stable association of CMG required all three components of the CMG complex as well as RecQL4, Ctf4/And-1, and Mcm10. Surprisingly, depletion of TopBP1, a homologue of Dpb11 that plays an essential role in the chromatin loading of Cdc45 and GINS in yeast cells, did not significantly affect CMG complex formation. These results suggest that the proteins involved in the assembly of initiation complexes in human cells may differ somewhat from those in yeast systems.T he initiation of eukaryotic DNA replication is a multistep process that requires the assembly of the prereplicative complex (pre-RC), activation of the pre-RC and formation of the replisome (1, 2). Pre-RC assembly occurs during the G 1 phase of the cell cycle by a stepwise recruitment of the origin recognition complex (ORC), Cdc6, Cdt1, and the Mcm2-7 complex onto DNA origins. During the G 1 /S transition stage, the recruitment of other replication factors such as Cdc45 and GINS and the combined actions of two S phase promoting kinases, the cyclindependent (CDK) and Cdc7-Dbf4 (DDK) kinases, lead to the assembly of an active DNA helicase complex at replication origins. This activation results in the unwinding of replication origins, the recruitment of DNA polymerases and accessory factors and the assembly of the replisome for DNA synthesis.In eukaryotic cells, the Mcm2-7 complex seems to be the catalytic core of the replicative helicase. It is essential for origin unwinding and replication fork progression (3-6). It has been shown that a variety of Mcm complexes, including the Mcm4/6/7 subcomplex (from many species), double hexameric complexes of Mcm homologues in Archaea and the budding yeast Mcm2-7 complex, exhibit DNA helicase activity in vitro (1, 7). In vivo, however, formation of an active helicase at replication origins requires the further recruitment of several factors including Cdc45 and GINS, and this activation process is governed by CDK and DDK (1).Both Cdc45 and GINS are required for the establishment and progression of the replication fork (8, 9). These proteins are loaded onto origins during S phase and form a stable complex with Mcm2-7 (10). The complex seems to be a DNA unwinding complex and moves along DNA as part of the replication fork complex (6, 11). Consistent with these observations, a complex of Mcm2-7, Cdc45, and GINS (the CMG complex), purified from D...