docs: def get_downregulated_genes(self) -> VertexSeq:
          """Get genes that are down-regulated.
  
          :return: Down-regulated genes.
          """
          down_regulated = self.graph.vs.select(self._is_downregulated_gene)
          logger.info(f"No. of down-regulated genes after laying on network: {len(down_regulated)}")
          return down_regulated