Regulatory sequences (e.g., transcription factor binding sites), unlike protein-coding regions, are subject to rapid turnover.
Challenges (in regulatory sequence identification) included the large non-coding search space in the human genome (∼98% of 3×109 bp), the small size and degenerate nature of transcription factor binding sites, and most importantly the lack of experimental training sets for computational methods to identify such sequences in a global manner.
上面是paper里头的原话。搞不明白这里的turnover和degenerate nature是不是一个意思？ 是指sequence本身会变化（mutation, etc.），还是说transcriotion factor is not binding to this cis element?
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