 Marcel Weber (2005: Ch. 7) begins his discussion of reference and conceptual change in genetics by drawing on Kitcher’s account of changing modes of reference and reference potentials, following Burian in emphasizing the important role of meaning in terms of descriptions or theoretical beliefs. Weber then presents a more fine-grained series of conceptual changes, driven by the introduction of new investigative techniques or new experimental organisms (esp. bacteria and phage), and observes that the reference potential (including theoretical beliefs) as well as the extension of the term “gene” changed continuously. Weber describes a situation where a broad extension based on the causal effects observed in Mendelian crossing experiments is narrowed down when additional criteria are introduced in classical, chromosome-based genetics, broadened again to include bacteria and viruses, and narrowed down again to sequences of DNA that determine the sequence of proteins (see  §§1.1–1.3).  Furthermore, in different contexts researchers could interpret the molecular characterization differently, for instance, including or excluding regulatory sequences.