PHOX2B is required for Ret expression in mouse pre-ENCDCs (154), and heterozygous PHOX2B polyalanine-expansion mutations cause congenital central hypoventilation syndrome (i.e., central sleep apnea) in humans, a syndrome that may include HSCR (Haddad syndrome) (2, 11). This evidence concerns the gene PHOX2B and Haddad syndrome.