The scope of CPS is continually widening thanks to many important chemical advances over the years. Peptide thioesters, the required starting materials for NCL, can be readily produced by common Fmoc-SPPS as masked thioester surrogates (most commonly hydrazides ). Segments can be assembled into proteins convergently using a combination of masked thioesters and removable Cys protecting groups (e.g., Acm, Thz, and Tfa-Thz ). Additional ligation sites beyond Cys have been unlocked by desulfurization reactions (originally converting Cys to Ala but expanded to many thiolated aa including the commercially available βthio-valine [penicillamine] ), as well as alternative ligation chemistries (e.g., serine-threonine ligation [STL], α-ketoacid-hydroxylamine ligation [KAHA], diselenide-selenoester ligation [DSL] , and others ). With a growing list of options in the modern protein chemist's toolbox, a wide variety of proteins are now accessible; on the other hand, every synthetic approach requires careful planning and consideration of available tools.