A method to determine which genes in a population of cells are expressed at a given time. Nascent RNA transcripts are radiolabelled with (³²P)NTPs as they are elongated in isolated nuclei (new transcripts are not initiated in these isolated nuclei). To determine if a specific gene is expressed, the radiolabelled run-off transcripts are used as hybridization probes with DNA from recombinant plasmids that contain the gene of interest.