In our hands, Brca1−/−53bp1−/− mice were born at around half of the expected frequency (Fig. 5a and Table 2) and weighed less than their Brca1+/−53bp1−/− littermates, with most succumbing to thymic lymphoma between 12 and 20 weeks of age, consistent with recent reports20,45. Here, BRCA1 is linked to thymus lymphoma.