 There are several virtues of the causal-mechanical approach to understanding scientific explanation in molecular biology. For one, it is truest to molecular biologists’ own language when engaging in biological explanation. Molecular biologists rarely describe their practice and achievements as the development of new theories; rather, they describe their practice and achievements as the elucidation of molecular mechanisms (Baetu 2017; Craver 2001; Machamer, Darden, Craver 2000). Another virtue of the causal-mechanical approach is that it captures biological explanations of both regularity and variation. Unlike in physics, where a scientist assumes that an electron is an electron is an electron, a biologist is often interested in precisely what makes one individual different from another, one population different from another, or one species different from another. Philosophers have extended the causal-mechanical account of explanation to cover biological explanations of variation, be it across evolutionary time (Calcott 2009) or across individuals in a population (Tabery 2009, 2014). Tabery (2009, 2014) characterized biological explanations of variation across individuals in a population as the elucidation of “difference mechanisms”. Difference mechanisms are regular causal mechanisms made up of difference-making variables, one or more of which are actual difference makers (see  Section 2.3  for the discussion of Waters’ (2007) concept of an actual difference maker). There is regularity in difference mechanisms; interventions made on variables in the mechanisms that change the values of the variables lead to different outcomes in the phenomena under investigation. There is also variation in difference mechanisms; interventions need not be taken to find differences in outcomes because, with difference mechanisms, some variables are actual difference makers which already take different values in the natural world, resulting in natural variation in the outcomes.