RB1 and neoplasm: The smallest region of overlap of the chromosome 14 losses encompassed Rb1 (Fig. 2b, XB04 and XB30, respectively), a key tumor suppressor gene whose loss has been found to be a common feature shared by BRCA1 deficient human breast and mouse mammary cancers29,30, suggesting a selection for a loss of Rb1 or another gene in the region.