Determination of Foreign Gene Copy Number in Stably Transfected Cell Lines by Southern Transfer Analysis

The quantitative appraisal of the number of foreign gene copies integrated within the genomes of stably transfected cells is most conveniently performed using the strategy known as Southern blotting (1 ,2 ). First introduced by E. M. Southern in 1975 (2 ) the basic protocol involves the following steps: Size-fractionated DNA is first transferred from a gel matrix to a solid support under conditions that prevent self-annealing. The DNA molecules are then immobilized (covalently linked to the support) and processed for hybridization to a radiolabeled probe that has a nucleotide sequence complementing that of the target sequence to be detected. The blot is then washed extensively to remove unreacted probe molecules, and the hybrids formed between the probe and target sequences are revealed by autoradiography. The relative intensity of each autoradiographic signal will reflect the amount of hybridized material present. Quantification may be performed visually, by scanning laser densitometry, or, if the hybrids are sufficiently radioactive, by liquid scintillation counting.