Get genes that are down-regulated.

        :return: Down-regulated genes.

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