How long should cells be fixed for my ChIP experiment?

Many protocols specify fixing cells in formaldehyde with a final concentration of 1% for 10 minutes at room temperature. If one does not observe any enrichment, but believes the protein should be there, they should increase crosslinking time to 15-20 minutes. If over crosslinking occurs, the same signal may be observed across different locations. While these fixation methods usually work for most sample types, some experiments may need adjustments to achieve optimal results.