Authors participating in the renewed discussion of germ-line gene therapy have begun conflating two senses of the term "prevention," which I distinguish as "phenotypic prevention" and "genotypic prevention." Phenotypic prevention describes medical efforts to forestall the clinical manifestation of a genetic disease in an at-risk patient, like newborn screening and dietary prophylaxis for phenylketonuria. Genotypic prevention, by contrast, describes efforts to avoid the transmission of particular genotypes to the next generation, like selective termination following intrauterine diagnosis. Genotypic prevention is either performed on behalf of a prospective parent (or two) as a reproductive risk reduction strategy, or as a public health intervention to reduce the incidence of a disease in the larger population. Conflating phenotypic and genotypic prevention in discussions of germ-line gene therapy is dangerous, because it blurs the line (well-established in clinical genetics) between medical interventions appropriate to prescribe to individuals and families, and reproductive choices that should be theirs alone to make. As the new genetic medicine emerges, its pioneers should be careful to articulate their professional goals in ways that respect that important moral boundary, by explicitly excluding genotypic prevention from among them.