The N-terminus of the Breast Cancer-1 predisposition protein (BRCA1) associates with the BRCA1-associated RING domain-1 protein (BARD1) to form a heterodimer, which exhibits ubiquitin ligase activity that is abrogated by known cancer-associated BRCA1 missense mutations. The majority of missense substitutions identified in patients with a personal or a family history of disease have not been followed in pedigrees, nor there is a functional understanding of their impact. We have examined, by extensive missense substitution, the interaction of BRCA1 with components that contribute to its ubiquitin ligase activity, BARD1 and the E2 ubiquitin-conjugating enzyme, UbcH5a. Selection from a randomly generated library of BRCA1 missense mutations for variants that inhibit the interaction with these components identified substitutions in residues found altered in patient DNA, indicating a correlation between loss of component-binding and propensity to disease development. We further show that the BRCA1:E2 interaction is sensitive to substitutions in all structural elements of the BRCA1 N-terminus, whereas the BARD1 interaction is sensitive to a subset of BRCA1 substitutions, which also inhibit E2-binding. Patient variants that inhibit the BRCA1:E2 interaction show loss of ubiquitin ligase activity and correlate with disease susceptibility and theoretical predictions of pathogenicity. These data link the loss of ubiquitin ligase activity, through loss of E2-binding, to the majority of non-polymorphic patient variants described within the N-terminus of BRCA1 and illustrate the likely significant role of BRCA1 ubiquitin ligase activity in tumour suppression.