tokens: Cellular fractionation and Percoll gradient centrifugation combined with immunoblotting show that p67 cofractionates with nuclei and is enriched in resistant structure that is insoluble in 2 M NaCl , 25 mM lithium 3 , 5 ' - diiodosalicylate , and 1 % Triton but is soluble in 8 M urea .