TWIST1 and craniosynostosis: Compound heterozygous Twist1(+/-)/Gsk3β(+/-) had a significantly higher frequency of craniosynostosis than predicted by the additive effects of each mutation alone (increasing from 35% in Twist1(+/-) and 15% in Gsk3β(+/-) to 94% in Twist1(+/-)/Gsk3β(+/-) compound heterozygotes).