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Epidermal growth factor (EGF) is a small protein containing 53 amino acids and three disulfide bonds. There is significant current interest in structure-function relationships in EGF and EGF-like proteins, including the homologous type-alpha transforming growth factors. The Leu-47 residue of murine EGF (mEGF) is one of several that are strongly conserved among the EGF-like growth factors, suggesting that it may contribute to the active site of mEGF. In several different binding assays, the activity of the mutant analog in which Leu-47 is replaced by Ser [( Ser47]mEGF) ranges from 8 to 18 times weaker than that of wild-type mEGF. The NMR data summarized in this paper demonstrate that the significant differences in the binding activities of wild-type and [Ser47]mEGF cannot be attributed to structural changes remote from the three-dimensional site of mutation. The only minor conformational changes that are indicated by these data involve side chains of residues proximal to Leu-47 in the three-dimensional structure. Therefore, Leu-47 and/or residues spatially adjacent to Leu-47 constitute part of the active site of mEGF.