During the first half century of genetics, coinciding with the first half of this cen tury, geneticists dreamt of the repair of genetic disease by altering or replacing defective genes. H. J. Muller wrote of the great advantages of mutations, "nanoneedles" in his apt term, for delicately probing physiological and chemical processes. In the same spirit, genes could be used to provide treatments of needle point delicacy. Yet, during this period no realistic possibility appeared; it remained but a dream. The situation changed abruptly at the half century. Microbial genetics and its offshoot,...