The present disclosure relates to a method of preparing exonuclease resistant molecules of block- decoys, use of the molecules in methods of modulating expression of recombinant proteins, particularly in vitro, for example by down regulation or inhibition of one or more transcription factors, and novel molecules of block-decoys, especially those obtained or obtainable from the methods herein. The disclosure also relates to use of said block decoys in vitro and in therapy. In one aspect there is provided a method for regulating recombinant gene expression in vitro comprising the steps of: a) providing a host cell encoding one or more recombinant genes for expression, b) contacting the cell with a exonuclease resistant block-decoy under condition suitable for the block-decoy to gain entry into the cell, and c) expressing the recombinant protein or protein.