One of the most important and technically difficult aspects of genomic sequencing is the electrophoretic transfer of the genomic DNA from the sequencing gel to the nylon membrane. Many different parameters have to be taken into consideration: electrical resistance, strength of the electric field, diffusion, Joule heating, stability of the buffer, etc. Most of the transfer systems described in the literature so far (Towbin et al., 1979; Gibson, 1981; McCellan and Ramshaw, 1981; Shuttleworth, 1984) do not fulfill the optimal conditions for genomic sequencing. A high resolution electrotransfer of the sequencing ladder requires an absolutely tight contact between the gel and the nylon membrane. Trapped gas bubbles (arising from electrolysis of the buffer) in the transfer system disturb the electric field and result in the distortion or loss of DNA bands in the blot. The electrical field has thus to be low enough to avoid buffer electrolysis, while at the same time being sufficiently high to ensure a quantitative diffusion-free transfer of the DNA.