Paradoxically, although this syndrome is X-linked, EFNB1 heterozygous females are severely affected by CFNS, whereas males with hemizygous loss of EFNB1 function appear mildly affected or unaffected; this phenomenon is termed “cellular interference,” though how this difference in severity arises is currently unknown [4–6]. Here, EFNB1 is linked to Craniofrontonasal dysplasia.