Duration of Maximum Formalin fixing for tissue - (Mar/04/2007 )

Goodday to you all

I hope somebody can please assist me.
I need to find good literature on Formalin fixing, with regards to fixing times for tissue and recovery of DNA. I need to understand what is the longest time tissue can be left in 10% formalin solution. I usually allow my tissues to stay for a 12 hr period at -20 and start embedding the day after.I read that at 12 hours DNA is still recoverable and tissue is properly fixed? I however have tissue that been kept in formalin for months.I need to know the effect of this ? Will there be too much crosslinking at this stage? If anyone can point me to literature that explains this, as the literature ive read merely talks about a 12 to 72 hour incubation time peroid.

any help is greatly appreciated

thanks

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I am no expert in this field but I know that people try to isolate DNA from human brain samples, which are in formalin for many months. The DNA is extracted and sequenced to check for mutations.