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DNA molecules less than ~20 kilobase (kb) pairs in size are efficiently fractionated by unidirectional electrophoresis in agarose gels (1). The variation of DNA mobility with gel concentration can be described by the Ogston mechanism of pore size distribution, if the observed mobilities are first extrapolated to zero electric field strength (2). Therefore, sieving of the macromolecules by the matrix appears to be the dominant mechanism of mol wt separation in this size range.