I am trying to optimize sonication for ChIP of AML cell lines viz. HL60 U937 THP1. I am fixing cells with formaldehyde and sonicating 2.5 million cells at a time. If anyone has worked with these cells before, it would be helpful if they share the conditions for efficient sonication for ChIP seq. Also, how much time do we need to keep cells in hypotonic buffer for lysis. Can it exceed 20 min?