Ubiquitination is characterised by covalent attachment of ubiquitin to a target protein. Attachment of a single ubiquitin to a substrate is called monoubiquitination. Polyubiquitin chains are the result of an elongation process in which substrate-attached ubiquitin itself serves as the substrate. Two ubiquitin proteins can be conjugated through the C-terminus of the incoming ubiquitin and one of the seven lysine residues (K6, K11, K29, K33, K48 and K63) or, alternatively, the N-terminal methionine residue (M1) of the ‘substrate’ ubiquitin. Linear ubiquitin chain assembly complex (LUBAC) is the only E3 ligase known to date that is capable of producing linear M1 ubiquitin linkages de novo. Linear ubiquitin chains are crucial modulators of innate and adaptive immune responses, and act by regulating inflammatory and cell death signalling.