the u03b1 position m nonprotein amino acids which are combinations of one or more of different or same alterations used in the a l and n nonprotein amino acids which differ in chemical structures from natural amino acids but can serve as substrates for aminoacyl trna synthetase by assuming conformation analogous to natural amino acids when bound to this enzyme cells to be cultured in a medium containing such nonprotein amino acids may be any suitable ones which produce proteins the term 34 protein 34 herein including glycoproteins such cells may be either wild type cells or transformants the term 34 transformants 34 herein including products by transfection and transduction consequently proteins to be produced by the cells according to the present invention can be those which wild type cells cannot produce in view of diversity of non natural proteins with nonprotein amino acids incorporated or alloproteins the proteins to be produced by the cells are preferably different from those produced by wild type cells in other words it is preferable that the cells to be used in the present invention be transformants representative cells according to the present invention are those of escherichia coli and transformants thereof and an example of the protein to be produced is a human epidermal growth factor hereinafter referred to.