The massXpert project aims at providing (bio)chemists with a software package allowing the following: * User-specific atom definitions and polymer chemistry definitions; * Powerful sequence editing with user-defined glyphs for each monomer and monomer chemical modification. Multi-region selections; * Polymer sequence chemical/enzymatic cleavage; * Intramolecular cross-linking, like disulfide bonds with total resolution of the peptides generated upon enzymatic cleavage; * Gas-phase fragmentation of oligomers; * Mass-to-charge ratio calculations with inline change of ionization agent; * Calculation of net electrical charge and of isoelectric point (even by taking into account monomer modifications if polymer is a protein); * Simulation of isotopic patterns for any chemical formula and z charge; * A number of plugins allow 1) translation of the sequence from one format to another (using a dictionary, like between the 1-letter code and the 3-letter code of proteins) 2) conversion of number format according to any localized number format 3) sort mass lists. WWW: http://massxpert.tuxfamily.org/