Transcriptionally Defective Retroviruses Containing lacZ for the In Situ Detection of Endogenous Genes and Developmentally Regulated Chromatin

Previously, we demonstrated that expression of transduced Escherichia coli lacZ can be detected in individual mammalian cells with a fluorescence-acti-vated cell sorter (Nolan et al. 1988), a technique re-ferred to as FACS-GAL. This fluorogenic assay per-mits the quantitative measurement of E. coli/3-galac-tosidase (/3-Gal) in individual viable cells. This tech-nique also permits rare cells expressing /3-Gal to be isolated utilizing the sorting capacity of the FACS. We have taken advantage of this feature to isolate/3-Gal-expressing cells where the lacZ gene is under the con-trol of endogenous transcription control elements. To permit the isolation of such cells, we have developed systems for delivering a reporter gene, E. coli lacZ, into the genome of mammalian cells in such a way that