PHOX2A and congenital fibrosis of the extraocular muscles: Biallelic PHOX2A LOF causes CFEOM in humans and absence of CN3/CN4 motor nuclei in mice.1 Monoallelic MAFB LOF causes DRS in humans and absence of CN6 motor nuclei in mice, with secondary aberrant innervation by CN3 of the lateral rectus muscle, which is normally innervated by CN6.2 While these and other mechanisms explain some oCCDDs, many remain genetically undefined.