The production of transgenic mice by the direct microinjection of DNA fragments into isolated mouse embryos is now a standard technique for molecular and developmental analysis (1). Traditionally, the initial screening of litters of mice for those carrying the transgene has been carried out by the preparation of DNA from pieces of excised tail obtained at 2-3 wk of age ("tail blot"). The presence of the transgene is determined through Southern blot analysis, a procedure that usually takes a minimum of 5 d. Two important criteria for successful screening are that the tail DNA is sufficiently purified from connective tissues and proteins to enable restriction enzyme digestion, and that the amount of DNA is large enough to allow the detection by hybridization of radioisotope-labeled probes recognizing the transgene or endogenous gene sequences.