Much has been learned about protein ubiquitination by studying the structural, biochemical, and biophysical properties of ubiquitin chains in vitro. However, these analyses were limited to K48-, K63-linked, and linear ubiquitin chains. Only recently, enzymatic and chemical assembly systems for the remaining chain types have been developed. Here, we describe a method to synthesise K11-linked ubiquitin chains in vitro by exploiting the intrinsic K11-specificity of the E2 enzyme UBE2S.