docs: def get_genes_for_hgnc_id(self, hgnc_symbol):
          """ obtain the ensembl gene IDs that correspond to a HGNC symbol
          """
          
          headers = {"content-type": "application/json"}
          
          # http://grch37.rest.ensembl.org/xrefs/symbol/homo_sapiens/KMT2A?content-type=application/json
          
          self.attempt = 0
          ext = "/xrefs/symbol/homo_sapiens/{}".format(hgnc_symbol)
          r = self.ensembl_request(ext, headers)
          
          genes = []
          for item in json.loads(r):
              if item["type"] == "gene":
                  genes.append(item["id"])
          
          return genes