The conserved armadillo repeat (ARM) domain of adenomatous polyposis coli (APC) protein plays an important role in the recognition of its binding partners. In this study, we report the crystal structure of ARC-ARM (residues 407-775), which was determined to 2.9 angstrom resolution. Our structure shows that the seven armadillo repeats of ARC-ARM fold together into a compact domain, with Arm2 and Arm5 presenting some deviations from canonical armadillo repeats. There is a positively charged groove on the surface of ARC-ARM, which might be the recognition site for ARC-binding partners. Comparison of this structure with our previously reported structure of ARC (407-751), together with normal mode analysis, reveals that the ARC-ARM domain possesses a limited intrinsic flexibility. We propose that this intrinsic flexibility might be an inherent property of ARM domains in general. (C) 2011 Elsevier Inc. All rights reserved.