To understand the control of gene expression in higher eukaryotes we must be able to: (a) easily distinguish active genes from inactive genes at the level of the nucleus, and (b) introduce genes with well-defined DNA sequences into cells and observe their expression. Experiments have been performed using both approaches. Pancreatic DNase has been successfully used to monitor the first-hit nucleolytic attack of genes in nuclei. Using this technique active genes can be easily distinguished from inactive genes. Also, we have been able to successfully introduce several genes into mouse Ltk cultured cells. The introduced genes are integrated into DNA carried in these cells and some are actively expressed, a process designated as biochemical transformation. The nature of this integration and expression is now being actively investigated.