Comparison of BRCT domains. (a) CLUSTAL-W alignment of the PARP-1 BRCT domain (PARP), the second BRCT domain of XRCC1 (X1BRCTb), and the first BRCA1 BRCT domain (BRCA1). Residues contributing to the BRCA1 phospho-serine binding site are underlined and in red font; dual repeat interacting residues in the tandem BRCA1 BRCT domains are underlined and in green font, and XRCC1 homodimer interface residues are underlined and in cyan font (adapted from ). (b) Stereo view of a superposition of the structure of the sixth BRCT domain of TopBP1 (magenta) and PARP-1 BRCT (cyan). (c) Close-up stereo view of the superposition of the BRCA1 BRCT domain phosphoserine binding site (magenta) and the homologous region from the PARP-1 BRCT domain (cyan). Residues interacting with the phosphoserine (orange) in the BRCA1 BRCT domain (magenta), as well as structurally homologous residues from the PARP-1 BRCT domain (cyan) are shown in stick representation.