We have used recombinant DNA techniques to construct DNA tumor virus hybrids carrying various chromosomal eukaryotic genes. These recombinant molecules have been introduced into cultured mammalian cells by infection, transfection or transformation. Such experiments allow us to identify regulatory sequences, such as those required for the heavy metal-inducible transcription of the mouse metallothionein-I gene; to overproduce useful gene products, such as human growth hormone; and to determine the effects of naturally occurring mutations, such as a deletion that causes a human thalassemia.