Question about gel shift assay

[SAI-Jinling Xie]

Hi, there,

     We've been doing gel shift assay. But we always got non-specific 
shifted band. We used more than 220bp PCR products as probes and crude 
cell extract from E.coli for shifting. A company recommends very short 
probes. I wonder if our probes are not short enough or some reasons else. 
And does impurity of the protein cause this problem? Do I have to purify the 
protein? If somebody happens to have the similar experience, would you 
please reply to this account? I would highly appreicite it. Thanks a lot 
in advance.

Jinling Xie