Figure 5. E2f6 and Bmi1 show a genetic interaction in the development of the axial skeleton.

(A) Alcian blue (cartilage) and alzarin red (bone) stainings of wild-type, Bmi1 mutant and E2f6;Bmi1 compound mutant mice at E18.5. The following skeletal transformations are depicted in the Bmi1 and E2f6:Bmi1 compound mutant mice: An extra piece of bone anterior to the first cervical vertebra (C1); the C1 vertebra is transformed into the second cervical vertebra (C2) as evidenced by the altered morphology; the seventh cervical vertebra (C7) is transformed into the first thoracic vertebra (T1) as shown by the presence of vertebrosternal ribs. The right panel shows the novel C5 to C6 transformation present in the E2f6 mutant and E2f6;Bmi1 compound mutant mice. This is evidenced by the presence of a piece of cartlidge on C5 instead of on C6. (B) Penetrance of axial skeletal transformations in E2f6;Bmi1 compound mutant embryos.