AXIN2 and Sagittal craniosynostosis: Based on the results of monochorionic twins, we demonstrated Axin2 (c.1181G > A, p.R394H, rs200899695) mutation led to haploinsufficiency with incomplete penetrance in female, and additional prenatal risk factors (intrauterine growth restriction and breech position) were indispensable to trigger the occurrence of sagittal craniosynostosis in this Chinese female monochorionic diamniotic twin family.