One of the main problems of writing Systems Biology Markup Language ( SBML) file by hand is the use of the Mathematical Markup Language (MathML) to describe all the mathematical portions of the model. MathML expressions are long, complex & therefore error-prone. To facilitate the editing of mathematical expressions, a small equation editor has been designed based on jex. The user can type the equation as free text, in a natural infix notation, that is subsequently transformed into valid MathML using the subset of MathML elements supported by SBML . The math editor also provides several drop-down menus to display the different elements that are already defined in the model & can be included in the equation (again using the name instead of the id to improve the readability of the equation). There is also the possibility to add new SBML elements compartments, species or parameters from within the math editor window. This avoids the necessity to go back to the main window if the declaration of an element has been forgotten.