Post-Translational Modifications (POTAMOS) Mass Spectrometry Calculator is a free web application that simplifies mass spectrometry calculations for post-translationally modified proteins.

It consists of three interacting parts:

• Calculator can predict all possible numbers of combinations between methylations, acetylations or phosphorylations present in any peptide provided a mass value is given.
• Histone Browser contains a collection of histone variants comprising of the most frequently studied organisms in the lab. It can calculate the mass of either the whole protein or a fragment of it. It is also possible to add any number of manually controlled post-translational modifications chosen from a limited repertoire of them. These are methylation, acetylation or phosphorylation.
• Protease Cleaver can cleave any aminoacid sequence by using a protease from a given list and then calculate the masses of the resulting fragments.