Difficulties in NGS assembly
* Sequencing errors: two reads that came from the same place in the genome often have mismatching sequences
  
   AGTGATTAGATCATAGTAGAG
            || |||||||||
            ATGATAGTAGAGGATAGAT
* Repetitive DNA (~ 5-20% of human DNA is repetitive):
           TTAGGGTTAGGGTTAGGGTTAGGGTTAGGG
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