BRCA1—a breast and ovarian cancer suppressor gene—promotes genome integrity. To study the functionality of BRCA1 in the heterozygous state, we established a collection of primary human BRCA1+/+ and BRCA1mut/+ mammary epithelial cells and fibroblasts. Here we report that all BRCA1mut/+ cells exhibited multiple normal BRCA1 functions, including the support of homologous recombination- type double-strand break repair (HR-DSBR), checkpoint functions, centrosome number control, spindle pole formation, Slug expression and satellite RNA suppression. In contrast, the same cells were defective in stalled replication fork repair and/or suppression of fork collapse, that is, replication stress. These defects were rescued by reconstituting BRCA1mut/+ cells with wt BRCA1. In addition, we observed ‘conditional’ haploinsufficiency for HR-DSBR in BRCA1mut/+ cells in the face of replication stress. Given the importance of replication stress in epithelial cancer development and of an HR defect in breast cancer pathogenesis, both defects are candidate contributors to tumorigenesis in BRCA1-deficient mammary tissue.

Abolhassani et al. show that CD70 deficiency is a novel cause of combined immunodeficiency and EBV-associated diseases, reminiscent of CD27 deficiency. CD70–CD27 interactions play a nonredundant role regulating humoral- and cell-mediated immunity in humans, especially for control of EBV.

Human MDA5 deficiency features recurrent severe human rhinovirus (HRV) infections. MDA5 serves a protective and nonredundant role in protecting against HRV in the respiratory tract, through its effects on virus sensing and triggering of antiviral IFN signaling.