Contents

15 March 1991
Vol 251, Issue 4999

About The Cover

Site-specific recombination in mammalian cells. Transient expression of a yeast recombinase (FLP) in mammalian cells will either excise specific chromosomal sequences or target integration of extrachromosomal DNA to specific chromosomal sites. In this photomicrograph, excisional recombination permanently activated a chromosomally integrated -galactosidase construct in the cells that contain a blue histochemical reaction product. See page 1351. [Photomicrograph by Stephen O'Gorman and Geoffrey Wahl]