During this period, which approximately corresponds to weeks 5–8 in human embryonic development, both Efnb1Δ/Y null hemizygous and Efnb1+/Δ mosaic heterozygous embryos exhibit changes in facial shape relative to control embryos, but the changes are more pronounced in mosaic heterozygous embryos, presaging the increased severity ultimately seen in heterozygous female CFNS patients. The gene discussed is EFNB1; the disease is craniofrontonasal syndrome.