a, Schematic representation of the strategy to generate the mutant allele. The rearranged allele contains the genomic region encompassing human exons 8-10 (pink) inserted into the equivalent region of the murine Atr gene (red exons). The SS mutation is indicated in E9. b, Visualization of the location and sequence homology of the humanized ATR protein in the region encoded in E8-E10. The full-length quimeric protein presents a 99.1% homology with murine ATR. c, PCR genotyping, d, RT-PCR of ATR with primers at E8 and E10 and e, ATR western blot, of littermate MEF lines (c-e). f, ATR western blot in human fibroblast lines 1BR3 (control) and F02-98 (Seckel), together with ATR+/+ and ATRS/S MEF.