There are several chemical methods for sequencing DNA. The classical method, introduced in 1977 by Maxam and Gilbert (1 ), uses four different chemical reactions to attack, modify, and open the heterocyclic bases at guanine (G), adenine and guanine (A + G), thymine and cytosine (T + C), and cytosine (C) in a radioactive end-labeled DNA fragment. The reactions result also in breakage of the glycosidic bond. A second reaction with hot piperidine is required to cleave the DNA backbone at these positions. The reaction products are thus analyzed by electrophoresis in four different lanes of a polyacrylamide gel. Several alternatives to the basic protocol were developed, as reviewed in (2 ).
