4-Methoxytrityl (Mmt) is a very acid-sensitive cysteine protecting group that can selectively be removed by 0.5-1.0% TFA in the presence of scavengers, e.g. TES. Protecting groups of the t-butyl type and S-Trt are left intact under these conditions. Cleavage and cyclization can be achieved concomitantly by adding iodine. It can be used in the solid-phase synthesis of cysteine-containing peptides, and further to selectively deprotect a thiol function in order to introduce a labeling group. Rijkers et al. employed p-methoxytrityl protection for obtaining S-palmitoylated peptides on-resin. The high acid lability of Cys(Mmt) was exploited by Roberts et al. to obtain a library of thioether-linked cyclic peptides. Arendt et al. employed Cys(Mmt) for the synthesis of peptides containing cysteic acid by selective deblocking and oxidation of the thiol moiety with performic acid. Mmt is fully stable to HOBt.