  Causal specificity has been analyzed by Lewis (2000). The basic idea is that a causal relationship between two properties is specific when many different values in a causal property bring about many different and specifically different values of a resultant variable (the causal relationships instantiate something like a mathematical function).  An on/off switch is not specific in this technical sense because the causal property has only two values (on and off). A dimmer switch is causally specific in this sense. Genes can be specific difference makers because many specific differences in the sequences of nucleotides in DNA result in specific differences in RNA molecules. This is not the case with many other actual difference makers, such as polymerases, which are more like on/off switches (with respect to differences in linear sequences). Biologists have discovered, however, the existence of other actual difference makers, besides genes and DNA, that are causally specific with respect to the linear sequences of processed RNA and polypeptides, to some degree at least. For example, in some cells splicing complexes called splicosomes actually differ in multiple ways that result in multiple, specific differences in the linear sequences of processed RNA molecules. 