In my in vitro Kinase assay using purified proteins, I have been seeing a band even in my negative control (i.e without the Kinase) and it is almost of the same intensity as the protein of interest. I have already tried a number of ways to get rid of the band by changing protein stock, re-purifying it, varying concentration of protein/ATP, etc and have also changed the buffers. Any help in this regard would be of great help.