UDP-GlcNAc acyltransferase (LpxA) catalyzes the first step of lipid A biosynthesis, the transfer of R-3-hydroxymyristic acid from acyl carrier protein to the 3′-hydroxyl group of uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). Lipid A, a phosphorylated glucosamine disaccharide carrying long-chain saturated fatty acids, is the hydrophobic anchor of lipopolysaccharide (LPS) to the outer monolayer of Gram-negative bacteria. Lipid A is a very potent stimulant of the human immune system. When present in the body at high concentrations during a Gram-negative bacterial infection, may cause shock and death by an "out of control" excessive immune reaction. Lipid A biosynthetic enzymes have no mammalian counterparts. Thus, they are attractive targets for the design of new antibiotics.