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The MRE11-RAD50-NBS1 complex plays a central role in response to DNA double-strand breaks. Here, we identify a patient with bone marrow failure and developmental defects caused by biallelic RAD50 mutations. One of the mutations creates a null allele, whereas the other (RAD50 E1035Δ ) leads to the loss of a single residue in the heptad repeats within the RAD50 coiled-coil domain. This mutation represents a human RAD50 separation-of-function mutation that impairs DNA repair, DNA replication, and DNA end resection without affecting ATM-dependent DNA damage response. Purified recombinant proteins indicate that RAD50 E1035Δ impairs MRE11 nuclease activity. The corresponding mutation in Saccharomyces cerevisiae causes severe thermosensitive defects in both DNA repair and Tel1 ATM -dependent signaling. These findings demonstrate that a minor heptad break in the RAD50 coiled coil suffices to impede MRE11 complex functions in human and yeast. Furthermore, these results emphasize the importance of the RAD50 coiled coil to regulate MRE11-dependent DNA end resection in humans., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Author(s). Published by Elsevier Inc. All rights reserved.)