10090
There is significant flexibility in the transcription start site , which causes variable spacing between the promoter elements and the start site ; this in turn causes differences in the length and sequence at the 5 ' end of RNA transcripts and can be important for gene regulation .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 None