SIX2 and craniosynostosis: We show that asymmetric distribution of Erg and Six2 is lost in Twist1+/−;Tcf12+/− mutants, and in a companion study that similar asymmetric distribution of Grem1+ progenitors is lost in Tcf12−/− mutants, consistent with bones meeting end-on-end and fusing in these craniosynostosis models58.