Southern blotting is a method whereby DNA fragments in the gel are denatured by soaking in an alkali solution, carried out of the gel, and transferred onto a membrane. After drying the membrane, the DNA is fixed irreversibly. The net result is a replica on the membrane of the DNA fragment pattern from the agarose gel. This technique is used to demonstrate B-cell clon- ality in blood and bone marrow down to the 1% level, though more reliably at the 5% level. The analysis is relatively nonselective and will detect novel rearrangements in relapse that were not seen at diagnosis. Modifications of the technique have been used to determine illegitimate switch recombinations and mutations of oncogenes.