Signals on Molecular Weight Bands? - (Jun/25/2008 )

I am curious about reasons why a molecular weight band might show up on immunblotting. I understand some protein MW markers are designed to show up, but mine usually doesn't. It has on 4 out of 20 gels I've run, usually with high background. Is this just insufficient washing? I do 3X 5 min with 1 quick before.

Insufficient blocking or too high a concentration of secondary.