The role of the GlcNAcβ1,2Manα- moiety in mammalian development. Null mutations of the genes encoding UDP-N-acetylglucosamine:α-3-d-mannoside β-1,2-N-acetylglucosaminyltransferase I and UDP-N-acetylglucosamine:α-d-mannoside β-1,2-N-acetylglucosaminyltransferase I.2 cause embryonic lethality and cong