Genome (from any organism) is fragmented using restriction enzymes, then run fragments on a gel (get smear for genomic dna and fragments for plasmids). Transfer gel results on a filter paper (nitrocellulose gel). You can take filter paper copy and hybridize with a labelled (radioactivity) ssdna or rna probe. When you add a probe, it will hybridize to a fragment making a duplex, wherever it hybridizes is the sequence of interest. Idea is if the gene is unique, somewhere you will get hybridization. If there are many genes, you get many bands. Took dna from two people and restricted it to create fragments, used a metal ion specific probe for hybridize. When this experiment was done, when we expose on film for 2 days, see several bands. If you compare both dnas, there is only one difference: an extra band. This means these genes are multi-family, so they have similar sequence.