The present invention is directed to a method to diversify the chemical composition of proteins produced in vivo comprising the step of disabling, particularly by mutagenesis, the editing function of one of its aminoacyl tRNA synthetases. The present invention is also directed to nucleic acid sequences encoding such mutated aminoacyl tRNA synthetases having their editing site mutated and capable of mischarging its cognate tRNA with a noncanonical amino acid. Also described herein is an improved method for obtaining transformed cells capable of synthetizing in vivo proteins comprising at least a noncanonical amino acid and their use for the production of such proteins.