The MCM2-7 complex is the putative replicative helicase that unwinds replication forks. While an S.erevisiae MCM hexameric complex consisting of one copy of each of the six subunits has been reconstituted in vitro from purified subunits (Davey et al. 2003), an active helicase can be assembled with three of the six MCM proteins, MCM4, MCM6 and MCM7, in human (Ishimi,1997), S. pombe (Lee and Hurwitz, 2001) and S. cerevisiae (Kaplan et al., 2003). However, this in vitro reaction may not represent the in vivo situation. In vivo experiments suggest that all six subunits are required at the replication fork (Labib et al., 2000; reviewed by Tye and Sawyer, 2000).