  The concept of actual difference making can be applied to molecular genetics as follows. In an actual cell, where a population of unprocessed RNA molecules differ with respect to linear sequence, the question arises: what causes these differences?  The answer is that differences in genes in the cell cause the actual differences in the linear sequences in the unprocessed RNA molecules, and also in populations of RNA molecules and polypeptides. Genes are not the only actual difference makers of the actual differences in the linear sequences of these molecules. And this brings us to the second causal concept, causal specificity. 