chAPTER 27 The Use of Flow Cytometry to Detect Transfected Gene Products Raymond Bqjdoso, David Sargan, Keith Ballingall, and Andrew Sanderson 1. Introduction Flow cytometry and fluorescence-activated cell sorting (FACS) are tech- niques of great power used to screen cells rapidly for expression of particular gene products. These techniques have been of general utility in identifying and selecting populations of cells of defined characteristics from body fluids and other natural sources, More recently they have received extensive atten- tion as methods for screening cell-surface expressed gene products in transfected cells. These methods rely on the indirect coupling of detector molecules, usually fluorochromes, to specific molecules on the target cells. This may occur through conjugation of the fluorochrome to the ligand of a receptor, or, as is more generally the case, through the use of fluorochrome- conjugated antibodies specific for the transfected gene product. Cells displaying specific surface ff uorescence following exposure to a flurochrome conjugate may subsequently be positively selected (or excluded) by FACS. Since cells are sorted individually, FACS is an ideal technique for picking up very rare events and for finding very minor subpopulations. In theory at least, the experimenter may recover a single cell of the desired phenotype from a relatively large population. However, the examination of single cells From: Methods m Molecular Biology, Vol. 7: Gene Transfer and Expression Protocols Edited by: E. J. Murray 0 1991 The Humana Press Inc., Clifton, NJ 361