code: def expressionLevelsGenerator(self, request):
          
          rnaQuantificationId = request.rna_quantification_id
          compoundId = datamodel.RnaQuantificationCompoundId.parse(
              request.rna_quantification_id)
          dataset = self.getDataRepository().getDataset(compoundId.dataset_id)
          rnaQuantSet = dataset.getRnaQuantificationSet(
              compoundId.rna_quantification_set_id)
          rnaQuant = rnaQuantSet.getRnaQuantification(rnaQuantificationId)
          rnaQuantificationId = rnaQuant.getLocalId()
          iterator = paging.ExpressionLevelsIterator(
              request, rnaQuant)
          return iterator
docs: Returns a generator over the (expressionLevel, nextPageToken) pairs
          defined by the specified request.
  
          Currently only supports searching over a specified rnaQuantification