 Waters (2007, see also his entry on  molecular genetics),  in reply, has argued that there is something causally distinctive about DNA. Causes are often conceived of as being difference makers, in that a variable (i.e., an entity or activity in a mechanism) can be deemed causal when a change in the value of that variable would counterfactually have led to a different outcome (see the entry on  scientific explanation).  According to Waters, there are a number of potential difference makers in the mechanisms involved in developing or not developing cystic fibrosis; that is, an individual with two normal copies of the gene could still display signs of cystic fibrosis if a manipulation was done to the individual’s RNA polymerase (the protein responsible for transcribing DNA to RNA), thereby undermining the functional reading of the stretch of DNA. So RNA polymerase is a difference maker in the development or lack of development of cystic fibrosis, but only a potential difference maker, since variation in RNA polymerase does not play a role in the development or lack of development of cystic fibrosis in natural populations. The stretch of DNA on chromosome 7, however, is an actual difference maker. That is, there are actual differences in natural human populations on this stretch of DNA, which lead to actual differences in developing or not developing cystic fibrosis; DNA is causally distinctive, according to Waters, because it is an actual difference maker. Advocates of the parity thesis are thus challenged to identify the other resources (in addition to DNA) that are actual difference makers.