 When applying theories of reference, philosophers typically commit to realism of some form. This means that they assume that there are natural kinds and that a concept, when it refers successfully, picks out some natural kind. When dealing with complex matters such as the phenomena of heredity, there are certainly several natural kinds to be identified. Hence, according to Kitcher, not only the reference potential of a concept changes, but also the reference of the concept itself. Earlier as well as later uses of the term “gene” might have referred successfully, but the extensions of the term in earlier and later instances might nonetheless have been different. Mendelians might have picked out the natural kind of, say, chromosomal elements that are potential difference makers with respect to heritable phenotypic variation when using the term. This kind comprises protein-coding DNA sequences and some non-coding regions. Molecular geneticists might have picked out another natural kind, say, template for functional molecules, which did not include some of the elements referred to as genes by Mendelians, but protein-coding DNA sequences as well as sequences coding for functional RNA. While this opens the door to a form of pluralism regarding genes as natural kinds, Kitcher connects his account to a notion of progress. He maintains that the reference potential of the gene concept improved. Descriptions that failed to fix reference were rejected and causal trajectories going back to the ostensive introduction of a term that turned out to have failed to pick out a natural kind or picked a natural kind no longer held to be significant are cut. In Kitcher’s view molecular genetics refined the concept of the gene. The notions of failure of reference and improvement of the reference potential are of course dependent on the status of molecular biological knowledge at a given time, which provides the baseline for the evaluation of concepts (Weber 2005: Ch. 7; for developments since Kitcher’s writing, see  §1.5).