def panel(self, panel_id):
        """Fetch a gene panel by '_id'.

        Args:
            panel_id (str, ObjectId): str or ObjectId of document ObjectId

        Returns:
            dict: panel object or `None` if panel not found
        """
        if not isinstance(panel_id, ObjectId):
            panel_id = ObjectId(panel_id)
        panel_obj = self.panel_collection.find_one({'_id': panel_id})
        return panel_obj