DNA is first digested by two restriction enzymes, labeled with radioisotopes, and subjected to the first gel electrophoresis in a noodle-shaped agar. DNA in the gel is subsequently digested by a third restriction enzyme (in-gel digestion) and the gel is laid on the top of a two-dimensional polyacrylamide gel for the second electrophoresis. Nearly 2,000 spots representing DNA fragments can be identified after autoradiography. Using a sophisticated computer image analysis system, mutations mainly caused by deletions or amplifications can be identified by comparison of images for both parents and the child. Single base substitutions at the recognition sites of the restriction enzymes can also be detected. This method has the advantage of being able to examine many genes at a time without any probes.