"DNA replication origins remain poorly defined in metazoans, in contrast to bacteria or S. cerevisiae. We believe that, in eukaryotes, a differential encoding of chromosomes by DNA replication origins is instrumental for the acquisition of cell identity during development. We wish to decipher this encoding, to determine whether it is linked to gene expression, and identify the mechanisms used to build a replication origin. First, we will unravel the origins code in mouse undifferentiated pluripotent cells with a genome-wide approach and correlate the replication origins' map with other chromosomal genetic or epigenetic features. The origins code will be deciphered also in the same cells when engaged into a specific (neural) differentiation. We predict that this differential mapping will identify constitutive and regulated origins, the latter specific to gene domains expressed in differentiation and providing cell identity. In the second axis of this project, we will identify proteins that constitute a replication origin. We will exploit two novel screening procedures developed in our laboratory. The "chromosome-trap" assay uses DNA beads to collect factors assembling at replication origins, using Xenopus cell-free systems. The "Replication foci capture" method allows the isolation of replication origins at their in-situ chromosomal location. Key objective 1: to unravel DNA replication origins' code in pluripotent embryonic stem cells. Key objective 2: to identify a link between replication origins and cell identity . Key objective 3: to investigate whether replication origins are responsible for the spatio-temporal organization and cell identity regulation by Homeobox domains. Key objective 4: to perform a functional analysis of replication origins by SiRNA silencing of differentially expressed gene domains or specific KO of DNA replication origins. Key objective 5: to identify new factors which assemble replication origins by two novel screening procedures."