The Breast Cancer Risk protein BRCA1 role is becoming analysed. While the mutation/deletion of this protein makes young women (males too) susceptible to develop breast tumours prematurely, how the protein did this was not clear for some time:

While Skully and collegues (doi: 10.1073/pnas.1400783111)  claim a role in replication fork control, Palazzo et al. (doi: 10.4161/cc.27945) observe it is related to the mitotic spindel formation. Chiba and collegue (DOI: 10.1016/j.molcel.2013.10.028) show a complex pattern of BRCA1, its dimerisation partner BARD1 and  the Obg-like ATPase 1, Ola-1 in the organisation of the centrosome.

Therefore, being involved in the basics of the machinery of cellular growth it is clear that BRCA1 once mutated makes the cell instable and prone to neoplastic transformation.