Chemical ubiquitination is an effective approach for accessing structurally defined, atypical ubiquitin (Ub) chains that are difficult to prepare by other techniques.H erein, we describe as trategy that uses ar eadily accessible premade isopeptide-linked 76-mer (isoUb), whichh as an N-terminal Cys and aC -terminal hydrazide,a st he key building blockt o assemble atypical Ub chains in am odular fashion. This method avoids the use of auxiliary-modified Lysa nd instead employs the canonical and therefore more robust Cys-based native chemical ligation technique.The efficiency and capacity of this isoUb-based strategy is exemplified by the cost-effective synthesis of several linkage-and length-defined atypical Ub chains,including K27-linked tetra-Ub and K11/K48-branched tri-, tetra-, penta-, and hexa-Ubs.