Positive control for Myc antibody

[fabianh from umich.edu]

Hi,
I will have a His-Myc tagged fusion protein which I want to detect on 
Western Blots with a purchased Myc antibody. What should I use as a 
positive control? (I don't have any other protein with Myc tag 
available!)I was told that for example Hela cell lysate would be a 
possible control but how is Myc expressed in these cells and what size 
will it have?
I am thankful for any ideas!
Fabian