SmaI digestion problems

[Rosemary Thresher]

I have been trying to digest DMF prepared mouse genomic DNA with SmaI to
use for Southern blots.  The digests always look incomplete.  Does anyone
know what the average fragment length of mouse DNA digested with SmaI, or
if there are any satellite bands created by SmaI.  At the moment I have
been attempting to digest 5ug of DNA in 30ul total volume, using 60 units
of enzyme (conc 40u/ul) at 25 degrees overnight.
Can anyone help?

Jodie Macoun