docs: def get_downregulated_genes_network(self) -> Graph:
          """Get the graph of down-regulated genes.
  
          :return Graph: Graph of down-regulated genes.
          """
          logger.info("In get_downregulated_genes_network()")
  
          deg_graph = self.graph.copy()  # deep copy graph
          not_diff_expr = self.graph.vs(down_regulated_eq=False)
  
          # delete genes which are not differentially expressed or have no connections to others
          deg_graph.delete_vertices(not_diff_expr.indices)
          deg_graph.delete_vertices(deg_graph.vs.select(_degree_eq=0))
  
          return deg_graph