Summary: Both replicative stress and DNA damage initiate cellular processes collectively termed the DNA damage response. These processes include activation of appropriate DNA repair mechanisms, cell cycle checkpoints, and in some cases, apoptosis. Accurate and efficient operation of the DNA damage response is essential for preventing mutations that may lead to oncogenic transformation or some types of inherited diseases. The DNA damage response involves sensing the damage, activation of specific kinases that transduce the activation signal via protein phosphorylation, and activation of effector proteins that carry out the functional aspects of the response. Two hallmarks of the DNA damage response are phosphorylation of key regulatory proteins and aggregation of multiprotein complexes into foci at or near the site of damage. The proteins that are phosphorylated and the composition of the foci depend upon the nature of the DNA lesion, and changes as the damage is recognized, processed and then repaired. Although different types of DNA damage activate specific repair proteins and pathways, some proteins respond to multiple types of lesions. Two protein complexes essential for the response to many lesions types are the Mre11/Rad50/Nbs1 (MRN) complex and replication protein A (RPA). Evidence supports the hypothesis that both of these complexes have multiple roles in the DNA damage response, including initial DNA damage recognition, activation of the signal transducing kinases and functional roles in DNA repair pathways. Although the MRN complex and RPA both become phosphorylated and form foci in response to multiple types of DNA lesions, we found that they co-localize to nuclear foci only in response to a subset of lesions. However, depletion of RPA via siRNA abrogates the ability of the MRN complex to form foci. These data suggest that the MRN complex and RPA have functional activities that can be both dependent and independent of each other. Understanding the determinant of whether or not the MRN complex and RPA interact, as well as the functional consequence of this interaction, will help elucidate the cellular responses to different types of DNA lesions and provide crucial information that may allow us to intervene to prevent the negative effects of DNA damage.