Mammary tumors in parous BK5.ATF3 mice. (a). Female wild-type mice (n = 12, closed squares) or BK5.ATF3 mice (n = 15, closed triangles) were allowed to mate and raise pups twice between 6 and 24 weeks of age, then observed until 16 months of age. Mice were euthanized when tumors reached 1.5 cm. Groups of nulliparous wild type (n = 20) and BK5.ATF3 (n = 13) females were also maintained for 16 months (data not shown); survival of both of these groups was 100% at the conclusion of the experiment. Sections of two representative tumors arising in BK5.ATF3 mice are shown at low power (b,c; scale bar = 200 μm); sections from five different tumors are shown at higher power (d-h; scale bar = 50 μm). KC, keratinaceous core.