Based on : expression of a biologically active product- functional cloning, detection of a physical molecule (either a specific seq. of dna. [hybridization] or a protein [immune detection, other metals- in vitro activity]) Hybridization: phage lyses of cells is more efficient to release the dna. Can do about 50,000 plaques per plate. Greater or lesser abilities of the probe to be detected. There is only one direction you can alter the stringency as you are washing (lower higher?) Everything is the same until : 5) immunodetection- instead of probe- we have antibody of interest, protein immunodetection (same protocol until we get to detection, 6) for dna: wash filter to remove unbound probe, dna detect by autoradiography. Now when we repeat, we will be able to line it up and identify individual plaques with the clone of interest (as opposed to knowing general areas and choosing between a bunch of plaques)