Sara,  The workshop I took at NSH stated the following fixatives
worked for RT-PCR, and they should also work for ISH
Crosslinking fixatives work
10% aq formalin, NBF, 4% paraformaldehyde, formal saline, zinc formalin

Avoid the following:

2% gluteraldehyde, crosslinks too much, for EM use 0.5%, is a crosslinker,
but they explained cannot be penetrated by PCR reagents.

Zenker, B-5, acetone/alcohols give poor morphology

Bouins

Heavy metal/picric acid fixatives cause DNA destruction.

Hope this helps,

Gayle Callis