If one consults any genetics textbook written before the mid-1970s, one will find much on the transformation of such bacterial species as Bacillus subtilis, but no mention of Escherichia coli. The method now most generally used for introducing DNA into E. coli is based on a study by Mandel and Higa in 1970 (1), who demonstrated that calcium chloride treatment would greatly enhance the ability of the cells to take up naked bacteriophage λ DNA. Later it was shown that the same method would also allow uptake of other DNA species including, in particular, plasmid DNA.