FAQ: When using the Monarch® Spin Plasmid Miniprep Kit (NEB #T1110), why might I observe a smear above the plasmid band on an agarose gel?

The presence of a smear at a higher molecular weight than the plasmid band may indicate that genomic DNA is contaminating the plasmid prep. While this is uncommon, genomic DNA can be sheared during lysis and co-purify with the plasmid. Vigorous shaking or vortexing after adding Buffer B2 will likely produce sheared genomic DNA and should be avoided. If the lysate is very viscous and does not mix well, start with fewer cells for the miniprep.
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