Dna from the organism extracted & cut into fragments with enzymes. Fragments separated into groups of fragments of different sizes (fractionated) by using electrophoresis. Fragments are blotted onto a piece of porous membrane where they maintain the same relative positions = southern blot -> probe for specific dna. Heat dna -> separate strands, the membrane is placed in a sol"n of the probe. The single-stranded probe will find & bind to its complementary dna sequence. On the blot, this binding concentrates the label in one spot -> relevant dna on gel is revealed. Find sets of genes -> use dna microarray -> genome wide probing ; array of dna fragments representing all the genes in a genome can be glued to the surface of a glass slide = microarray. Probes used are usually complex mixtures made by converting mrnas from one tissue into dna (cdna).