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 protecting groups of the t-butyl type and S-Trt. It can be used in the solid-phase synthesis of cysteine-containing peptides, and further to selectively protect a thiol function in order to introduce a labeling group.