Fig. 8. Model of the function of NOPO in the early embryo. We propose a model in which BEN-UEV1A-NOPO (E2-E3) complexes ubiquitinate unidentified target proteins to ensure the maintenance of genomic integrity in the early Drosophila embryo. In the absence of NOPO-mediated ubiquitination, truncation of S-phase and/or spontaneous DNA damage result in mitotic entry with unreplicated and/or damaged DNA, respectively. A CHK2-mediated DNA checkpoint is then triggered that causes mitotic arrest and a block in embryonic development.