AbstractReplication protein A (RPA) is the major eukaryotic single stranded DNA binding protein that plays a central role in DNA replication, repair and recombination. Like many DNA repair proteins RPA is heavily phosphorylated (specifically on its 32 kDa subunit) in response to DNA damage. Phosphorylation of many repair proteins has been shown to be important for their recruitment to DNA damage‐induced intra‐nuclear foci. Further, phosphorylation of H2AX (γ‐H2AX) has been shown to be important for either the recruitment or stable retention of DNA repair proteins to these intra‐nuclear foci. We address here the relationship between DNA damage‐induced hyper‐phosphorylation of RPA and its intra‐nuclear focalization, and whether γ‐H2AX is required for RPA's presence at these foci. Using GFP‐conjugated RPA, we demonstrate the formation of extraction‐resistant RPA foci induced by DNA damage or stalled replication forks. The strong DNA damage‐induced RPA foci appear after phosphorylated histone H2AX and Chk1, but earlier than the appearance of hyper‐phosphorylated RPA. We demonstrate that while the functions of phosphoinositol‐3‐kinase‐related protein kinases are essential for DNA damage‐induced H2AX phosphorylation and RPA hyper‐phosphorylation, they are dispensable for the induction of extraction‐resistant RPA and RPA foci. Furthermore, in mouse cells genetically devoid of H2AX, DNA damage‐induced extraction‐resistant RPA appears with the same kinetics as in normal mouse cells. These results demonstrate that neither RPA hyper‐phosphorylation nor H2AX are required for the formation in RPA intra‐nuclear foci in response to DNA damage/replicational stress and are consistent with a role for RPA as a DNA damage sensor involved in the initial recognition of damaged DNA or blocked replication forks. J. Cell. Biochem. 99: 1452–1462, 2006. © 2006 Wiley‐Liss, Inc.