Abstract: Maintenance of genome integrity relies on surveillance mechanisms that detect and signal arrested replication forks. Although evidence from budding yeast indicates that the DNA replication checkpoint (DRC) is primarily activated by single-stranded DNA (ssDNA), studies in higher eukaryotes have implicated primer ends in this process. To identify factors that signal primed ssDNA in Saccharomyces cerevisiae, we have screened a collection of checkpoint mutants for their ability to activate the DRC, using the repression of late origins as readout for checkpoint activity. This quantitative analysis reveals that neither RFC(Rad24) and the 9-1-1 clamp nor the alternative clamp loader RFC(Elg1) is required to signal paused forks. In contrast, we found that RFC(Ctf18) is essential for the Mrc1-dependent activation of Rad53 and for the maintenance of paused forks. These data identify RFC(Ctf18) as a key DRC mediator, potentially bridging Mrc1 and primed ssDNA to signal paused forks.