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]. The gene discussed is EFNB1; the disease is craniofrontonasal syndrome.