def is_valid_selection(ext):
    """ Checks if selection is correct """
    formats = ['System', 'Protein', 'Protein-H', 'C-alpha', 'Backbone',
        'MainChain', 'MainChain+Cb', 'MainChain+H', 'SideChain',
        'SideChain-H', 'Prot-Masses', 'non-Protein', 'Water', 'SOL',
        'non-Water', 'Ion', 'NA', 'CL', 'Water_and_ions']
    return ext in formats
