SMAD6 and craniosynostosis: Additionally, haploinsufficiency of the BMP-specific inhibitory Smad SMAD6 is predicted to be a modifier of the penetrance of craniosynostosis, with mutations in SMAD6 increasing the risk of developing craniosynostosis (Timberlake et al., 2016; Timberlake et al., 2017; Timberlake et al., 2018; Calpena et al., 2020; Wu et al., 2020).