question about fixation

[Guan Bin]

I'm a student from Fudan University, and I am doing some in situ 
Hybridization in zebrafish embryo. The following is my question 
about fixation.  

Since PFA's role is to kill RNAse, so fix for longer time (such as 
Overnight) at Room Temperature is better. My in situ has proved it. 
But why your protocol suggest fix O/N at 4 =B0C?  

I'm looking forward your reply soon.

Thanks.

Guan Bin