In order to study the replication program in higher eukaryotes we use the Xenopus model system which allowed in the past to identify a great number of replication and checkpoint factors. This in vitro system, which consists of isolated sperm nuclei replicating efficiently in egg extracts, mimics early embryonic cell cycles with very short S phases. During the first rapid divisions in Xenopus embryos transcription is repressed and the ratio of nuclei to cytoplasm (nuclear-cytoplasmic ratio) increases therefore progressively. At the midblastula transition (MBT) between the 12.-13. division, the S phase lengthens, the G1 and G2 phases of the cell cycle are introduced and widespread zygotic transcription initiates. S phase in differentiated cells takes several hours. It has been proposed that the spatial and presumably also the temporal replication pattern changes at the mid-blastula transition in Xenopus. In the Xenopus in vitro system where S phase lasts about 30 min we and others have shown that replication origins are spaced 5-15 kb and are clustered in early and late firing groups of origins (Marheineke and Hyrien 2001, Marheineke and Hyrien 2004). It has also been shown that several replication factors are titrated by the increase of the nucleo-cytosolic ratio after 12 divisions during early development which triggers the DNA replication checkpoint, slows down S phase and which would trigger the mid-blastula transition (MBT). Our lab investigates the mechanisms of the replication program in this developmental context. We have recently shown that shortly after the MBT the number of replication eyes decreases genome wide about 30% (Platel et al, 2019) (figure 2).