Cells do not lyse in SDS buffer - (Jul/01/2010 )

Dear ChIP-friends,

I am facing a big problem. I do ChIP Assays with HeLa cells which are infected with Adenovirus. Recently, the problem arose that the infected HeLa cells do not lyse in SDS-buffer anymore. They aggregate in kind of colonies and no lysis method causes them to break. Has anyone ever faced this problem and can provide any useful hint? What causes cells to prevent from lysis?

Thanks Nicole

I do not do ChIP but I do work with HeLa cells to do normal co-IP experiments and I usually lyse them with RIPA or 2% CHAPS buffers. why do you use only SDS buffer?