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The hexosamine biosynthetic pathway, whose end product is UDP-N acetylglucosamine (UDP-GlcNAc), lies at the base of cellular glycosylation pathways, including glycosylation of lipids, formation of heparin sulfated proteoglycans, and N- and O-linked glycosylation of proteins. Forward genetic studies in Drosophila have revealed that mutations in genes encoding different enzymes of the hexosamine biosynthetic pathway result in reduction of UDP-GlcNAc to different extents, with a consequent disruption of distinct glycosylation pathways and developmental processes. A maternal and zygotic loss-of-function screen has identified mutations in nesthocker (nst), which encodes an enzyme in the hexosamine biosynthetic pathway. Embryos lacking maternal and zygotic nst gene products show defective O-GlcNAcylation of a fibroblast growth factor receptor (FGFR)–specific adaptor protein, which impairs FGFR-dependent migration of mesodermal and tracheal cells.