 Tudor Baetu builds on the contemporary philosophy of mechanism literature as well to provide an account of how different experiments in molecular biology move from finding correlations, to establishing causal relevance, to establishing constitutive relevance (Baetu 2012b). Much recent philosophical attention has been given to the transition from correlation to causal relevance. On a manipulationist account of causal relevance, some factor X is determined to be causally relevant to some outcome Y when interventions on X can be shown to produce the change in Y. Experiments from molecular biology often take this form, which Baetu calls “one-variable experiments” because they involve a single intervention made on X in the experiment to establish the causal relevance to Y. But these one-variable experiments, Baetu cautions, do not necessarily provide information about the causal mechanism that links X to Y. Is X causally relevant to Y by way of mechanism A, mechanism B, or some other unknown mechanism? Baetu, drawing on his own experimental research in molecular oncology, shows that scientists probe the mechanical link by performing “two-variable experiments”. In a two-variable experiment, two interventions are simultaneously made on the initial factor and some component postulated in the mechanical link, thereby establishing both causal and constitutive relevance. 