Current drug discovery strategies include both molecular and empirical approaches. The molecular approaches are predominantly hypothesis-driven and are referred to as target-based. The empirical approaches are referred to as phenotypic because they rely on phenotypic measures of response. A recent analysis revealed the phenotypic approaches to be the more successful strategy for small-molecule, first-in-class medicines. The rationalization for this success was the unbiased identification of the molecular mechanism of action (MMOA).