Eukaryotes often contain multiple genes for a-and for b-tubulin, and express these different isotypes in different tissue-and developmental patterns . In order to study tubulin expression in an accessible differentiated cell model, we chose chicken erythrocyte tubulin, a tubulin previously reported to be fairly free of PTM . Purified chicken erythrocyte tubulin (ChET) was chemically cleaved using CNBr, and the most acidic peptides (which are the Carboxyl Terminal (CT) peptides) were purified by anion exchange chromatography at pH 6. The resulting peptides were examined by negative ion MALDI-MS; a representative spectrum is shown in Figure . Peptides were assigned according to the predicted m/z values of ChET isotypes of a-and b-tubulin or expected m/z values for known PTMs combined with these isotypes. A list of isotype masses of the Cterminus expected as a result of cleavage by CNBr is shown in Supplemental Table .