Consequently, we conclude that Rad17p-Rad17p interaction can occur independently of the Rad17p⅐Mec3p⅐Ddc1p complex and participation of Rad17p in two separate, possibly competing complexes can be assumed. One wonders if Rad17p might be able to form PCNA-like homotrimeric structures as originally suggested (40). However, MEC3 expressed from a multicopy plasmid does neither enhance nor reduce Rad17p-Rad17p interaction and thus no indication for a competition between different Rad17p-containing complexes was found (data not shown). Alternatively, the **data** are also consistent with a simultaneous complex formation of Rad17p with itself and Mec3p. One could envision multimeric aggregate formation of a PCNA-ring-like Rad17p⅐Mec3p⅐Ddc1p complex on DNA, mediated by Rad17p homomeric interactions. We currently favor this hypothesis because Rad17p but also Mec3p self-interactions have been indicated for the human homologues (hRad1, hHus1, Ref. 46), thus possibly hinting at an evolutionary conserved capability of the heterotrimer to form higher order complexes. On the other hand, protein elution profiles in S. pombe did not indicate any shift in Hus1 protein association following DNA damage by ionizing radiation (44). However, all of the complexes detected in this study were larger in size than the predicted single heterotrimeric complex.