Vitamin K, a fat-soluble vitamin, is required for γ-carboxylation of glutamic acid residues on precursors of vitamin K–dependent coagulation proteins (factors II, VII, IX, X, C, and S). Vitamin K exists in 2 forms: vitamin K1 or phylloquinone (the plant form of the vitamin) and vitamin K2, a series of compounds synthesized by bacteria and referred to as menaquinones. In contrast to human milk, infant formula contains large amounts of vitamin K (10 vs 65–100 mcg/L). Newborn infants are at risk for vitamin K deficiency because of vitamin K's poor placental transfer, insufficient endogenous production from the intestinal bacterial flora prior to complete colonization of the neonatal colon, and inadequate dietary intake among solely breast-fed infants.