During late mitosis and early G1, replication origins are licensed for 
subsequent replication by loading heterohexamers of the mini-chromosome 
maintenance proteins (Mcm2-7). To prevent re-replication of DNA, the licensing 
system is down-regulated at other cell cycle stages. A small protein called 
geminin plays an important role in this down-regulation by binding and 
inhibiting the Cdt1 component of the licensing system. We examine here the 
organization of Xenopus Cdt1, delimiting regions of Cdt1 required for licensing 
and regions required for geminin interaction. The C-terminal 377 residues of 
Cdt1 are required for licensing and the extreme C-terminus contains a domain 
that interacts with an Mcm(2,4,6,7) complex. Two regions of Cdt1 interact with 
geminin: one at the N-terminus, and one in the centre of the protein. Only the 
central region binds geminin tightly enough to successfully compete with 
full-length Cdt1 for geminin binding. This interaction requires a predicted 
coiled-coil domain that is conserved amongst metazoan Cdt1 homologues. Geminin 
forms a homodimer, with each dimer binding one molecule of Cdt1. Separation of 
the domains necessary for licensing activity from domains required for a strong 
interaction with geminin generated a construct, whose licensing activity was 
partially insensitive to geminin inhibition.