Bacteria and bacteriophages are in a constant evolutionary arms race, developing a variety of defence and counter-defence strategies. Recently, bioinformatic and functional screening has revealed more than a hundred new antiviral defence systems encoded in bacterial genomes. Understanding the action and inhibition of these systems at the molecular level may pave the way for the development of new molecular tools or effective antibacterial agents. However, only a few of them have been functionally characterised. The widespread bacterial defence system BREX consists of a gene locus encoding proteins that prevent phage infection by an as yet unidentified mechanism. The proposed PhD project will focus on the functional characterisation of BREX-associated defence proteins to disclose the molecular mechanisms of BREX protection and its inhibition.