DNA transfection has become an important technique in molecular biology. The fusion of putative gene-regulatory sequences to reporter genes and their introduction into a eukaryotic cell can be used for a detailed analysis of cisacting DNA regulatory sequences (1  ). DNA regulatory elements such as those responsive to CAMP (2  ) or to glucocorticoids and antiglucocorticoids (3  ) as well as to drugs and xenobiotics such as phenobarbital (4  , 5  ) can be identified through the use of approprtate reporter-gene constructs. Cotransfection experi-ments with vectors expressing specific transcription factors can be used to iden-tify the role of these transcription factors in the expression of specific genes (6  ) and in addition, the developmental activation of a promoter can be examined (7  ). The expression of functional protein in transfected cells can be useful for studies on genetic toxicology (8  ) or biochemical pathways (9  ). By the intro-duction of mutated DNAs that produce modified protems in the transfected cell, the functional significance of protein domains (10  ) and individual aminoacid residues (11  , 12  ) can be elucidated, and the transfection of cDNAs isolated from patients has already proved useful for the analysis of molecular defects that result in genetic disorders (13  ).