BARD1 and cancer: BRCA1 in complex with BARD1 exerts an E3 ubiquitin ligase activity that was once thought to be essential for tumor suppression, but this view was contradicted by a recent study of a transgenic mouse model homozygous for an engineered mutant Brca1 gene that is defective for ubiquitin ligase activity but retains the ability to mediate HDR, since these mice did not develop cancer.