The present invention relates to the use of a thermophilic nuclease for degrading nucleic acids in vivo and/or in situ, wherein the thermophilic nuclease is heterologous to the host cell and is produced by the host rather than being added exogenously. The present invention further relates to a genetically modified cell which was produced according to the above method. The present invention is particularly beneficial in inactivating the biological activity of recombinant DNA in biomass or biomass-derived products.