Elements involved in the molecular pathway of sutural osteoblast differentiation (modified from ref. 15). Top: normal cellular differentiation and relationship of TWIST and FGFR signaling. Bottom: altered gene expression in TWIST Y103X mutant osteoblasts from a Saethre-Chotzen syndrome patient and in FGFR2 S252W mutant osteoblasts from an Apert syndrome patient. Bone sialoprotein is a mature osteoblast marker.