Chromatin (chromosomal nucleoprotein) from mammalian colls is used as a template for the synthesis of RNA which is characterized and compared with other RNA by RNA-DNA hybridization. It is found to be transcribed from a restricted set of sequences and cannot be distinguished from natural RNA from the same organ as the chromatin. In contrast, it is different from RNA from other organs. Hence, DNA is masked in an organ-specific way in vivo and the masking is preserved on isolation. When cell division is induced in mouse liver and kidney a very early event is a change in masking in chromatin. This precedes changes in RNA populations; both precede DNA synthesis. Chromatin can be accurately reconstructed from DNA, histones and non-histone proteins. Experiments using this system indicate that histones non-specifically mask DNA; non-histone proteins are essential to reverse masking in a specific way.