DNA Replication Licensing Factor (RLF) is an essential activity required to restrict the duplication of genomic DNA to precisely once per cell cycle. Recent fractionation of RLF activity from Xenopus egg extracts has resulted in the identification of two essential components, RLF-B and RLF-M. RLF-M has been purified to homogeneity and has been shown to consist of a complex of proteins in the MCM/P1 family. RLF-B is still unidentified, but possible candidates for this activity have been identified in yeast. Elucidation of the RLF mechanism will provide important insights into the way that chromosome replication is controlled.