def onSelChange(self, selection):
        mxd = arcpy.mapping.MapDocument("CURRENT")
        theLayer = arcpy.mapping.ListLayers(mxd)[0]
        with arcpy.da.SearchCursor(theLayer, ["RR_UNIQ_ID"],""""RR_TYPE_NM" = '{0}'""".format(selection)) as sc:
            for row in sc:
                cmbFilteringCombo.items.append(row[0])
        cmbFilteringCombo.enabled=True