protein expression with and without induction - (Jul/05/2014 )

Hi, 

I'm trying to express a protein in E. coli BL21 (DE3) and I'm using the pET vector system.  what happened is that I performed two shake flask fermentations (500 mL LB each) starting from an overnight pre-culture. anyway, one fermantation was induced with IPTG (1 mM) and the other one was not.  after SDS-PAGE analysis of the 1/OD samples it was noticed that the protein was expressed in both fermentations and the OD measurements were almost the same and I actually cannot find an explannation for this 

Hope someone could help me explain this 

thank you 

It could be due to leaky expression. You could check this thread: http://www.protocol-online.org/biology-forums/posts/37042.html