Calculating Tm for mismatch on Northern

[neale at mbcf.stjude.org]

We have a partial cDNA clone from mouse that we wish to screen *human* northern
blots. So far, under our usual stringency (50% formamide, 6xSSC, 1% SDS, 42 C)
of hybridization we see no signal to very weak signal in the human samples.

My question is whether you can calculate a lower stringency and wash of these
blots the same as you would for Southerns? My first instinct is that I can, but
maybe I overlooking something.

Thanks,

Geoff