def colocalization_widget(self):
     controller = self.controller
     cases = Textarea(description='* Cases to co-localize:')
     by_signature = Checkbox(description='Cases indicated by signature?', 
                                           value=self.by_signature)
     slice_variables = Textarea(description='* Slice variables:')
     if 'biological_constraints' not in controller.options:
         bio_constraints = ''
     else:
         bio_constraints = ', '.join(controller.defaults('biological_constraints')) 
     constraints = Textarea(description='Biological constraints:',
                                          value=bio_constraints)
     button = Button(description='Create Co-Localization')
     button.on_click(self.make_colocalization)
     button.cases = cases
     button.by_signature = by_signature
     button.slice_variables = slice_variables
     button.constraints = constraints
     wi = VBox(children=[cases,
                              by_signature,
                              slice_variables,
                              constraints,
                              button])
     return ('Co-localizations', wi)