The regulation of expression of the genome is fundamental to elucidation of eukaryotic cell function espically during differentiation or in abnormal growth patterns such as cancer. In both cases, growth patterns are ultimately dependent upon selective transcription of portions of the genome. One important aspect of this nuclear activity is the synthesis and turnover of heterogeneous nuclear RNA by all higher eukaryotes examined to date. The apparent anomalous behavior of this RNA, i.e., synthesis of very large molecules containing considerable potential genetic information, of which are almost immeditely degraded without leaving the nucleus, must be a key to elucidating gene expression and regulation. We propose to examine the enzymatic basis for this degradation, the selectivity involved, the fate of the degradation products and hopefully, reasons for this extensive synthesis and breakdown.