MCM2-7 proteins are conserved replication factors functioning as DNA helicases during DNA synthesis. MCM8 is another member of this family, which appears to be specific for higher eukaryotes, as it is absent in worms and yeast. Here we report the complete identification of a novel member of this family, the MCM9 protein. Like MCM8, MCM9 is only present in the genome of higher eukaryotes. This protein contains an MCM8-like ATP binding and hydrolysis motif implicated in helicase activity. Strikingly, MCM9 also contains a unique carboxy-terminal domain which has only weak homology to MCM2-7 and MCM8 but is conserved within MCM9 homologs. We also show that the very recently reported human MCM9 protein (HsMCM9), which resembles a truncated MCM-like protein missing a part of the MCM2-7 signature domain, is an incomplete form of the full length HsMCM9 described here. Searching the human genome with either the newly identified human MCM9 or other MCM protein sequences, we did not detect further additional members of this DNA helicase family and suggest that it is constituted of eight members, falling into two different groups, one constituted by the MCM2-7 complex and the other by MCM8 and MCM9, which are present only in higher eukaryotes.