As was shown in earlier experiments in which isolated cell nuclei were fractionated, part of the nuclear proteins remained in the insoluble fraction after deoxyribonucleoprotein has been removed by high salt extraction. 52 , 53 , 77 A combination of biochemical and cytochemical analyses has revealed that the residual material consists of nuclear membranes, nucleoli, and some extranucleolar material scattered throughout nuclei. 78