Large contrasts are observed between the messenger RNA populations of different tissues and of embryos at different stages of development. Nevertheless, coding sequences for genes not expressed in a cell appear to be present in its nuclear RNA. Though many nuclear RNA transcripts of single copy DNA sequences are held in common between tissues, an additional set, probably consisting of non-message sequences, is not shared. Nuclear RNA also contains transcripts of repetitive DNA sequences. Certain repeat families are represented at high levels in the nuclear RNA of particular tissues and much lower levels in others. It is surprising that both complements of most repeat sequences are present in nuclear RNA. These observations lead to model for regulation of gene expression in which the formation of repetitive RNA-RNA duplexes controls the production of messenger RNA.