SmaI site gone!?

[T.J. Young]

Dear All,
A strange thing has happened.  I subcloned a cDNA sequence into a eukaryote 
expression vector using flanking SmaI restriction sites. I've used this 
construct for expression and everything seems to be working fine.  However, 
I've done some SDM on the gene in this vector and I now want to subclone it 
into a different plasmid.  Well, seeing as I ligated it in at SmaI sites I 
thought I would excise it at the SmaI sites but they appear to have gone!?  
Now the only thing I can think of is that I had some exonuclease activty 
arround when I did the ligation which has chewed back the SmaI recognition 
site.  Is this something other people have seen? Or could there be another 
explanation?  Thanks in advance.
Tim Young #8-)