<p> To be effective as a mechanism that preserves genomic integrity, the DNA damage checkpoint must beextremely sensitive in its ability to detect DNA damage. In <taxon tax_id="4932">Saccharomyces cerevisiae</taxon> the Ddc1/Rad17/Mec3 complex and Rad24 are DNA damage checkpoint components which may promote checkpointactivation by "sensing" DNA damage directly [<cite idref="PUB00007683"/>]. Rad24 shares sequence homology with RF-c, a protein that recognises DNA template/RNA primer hybrids during DNA replication. TheDdc1 complex has structural homology to proliferating-cell nuclear antigen (PCNA), which clamps ontoDNA and confers processivity to DNA polymerases delta and epsilon. Rad24 is postulated torecognise DNA lesions and then recruit the Ddc1 complex to generate checkpoint signals. </p>