'dna' - unmasked genomic DNA sequences.
 'dna_rm' - masked genomic DNA.  Interspersed repeats and low
  complexity regions are detected with the RepeatMasker tool and masked
  by replacing repeats with 'N's.
 'dna_sm' - soft-masked genomic DNA. All repeats and low complexity regions
  have been replaced with lowercased versions of their nucleic base
  

即，在ensembl数据库中，对于参考基因组中重复序列的标记，一般小写代表重复参考基因组中的重复序列。