4% PFA in PBS (pH 7.4)

Reagent	Final concentration	Amount
Paraformaldehyde	4% (w/v)	4 g
PBS (10×, pH 7.4)	1×	10 mL
ddH2O	N/A	up to 100 mL
Total	N/A	100 mL

Dissolve the paraformaldehyde in 50 mL ddH₂O by heating and stirring the solution up to 60°C (do not boil) and by adding 5 N NaOH drop by drop until a clear solution is formed. Add 10× PBS, let the solution cool down, adjust the pH to 7.4 and adjust the volume to 100 mL with ddH2O. Filter the solution, aliquot and freeze the aliquots at −20°C for long-term storage. Avoid repeated thaw-freeze cycles.