Slow DNA samples

[TR]

Hallo everybody,

Lately our DNA fragments seemed to be shorter than they should. It turned
out that our DNA samples are running slower that our markers!, from the very
begging of the run (see photograph at *
http://webpages.ull.es/users/cglez/CQSPB/gel.jpg*<http://webpages.ull.es/users/cglez/CQSPB/gel.jpg>,
the markers are in the central lane).

Any idea what the cause may be?

Thanks, CG