 One key challenge for reduction stems from the fact that the effect of a molecular entity or mechanism may depend strongly on the context in which it occurs (Hull 1972, 1974, 1976; see also Wimsatt 1979, Gilbert and Sarkar 2000, Laubichler and Wagner 2001). Classical geneticists were fully aware of the fact that a phenotype is brought about by the interaction of several (many) classical genes—the same allele may lead to two different phenotypes if occurring in two individuals with a different overall genotype (Waters 2004). Although one-many phenomena have been known for some time, it is now a robust empirical generalization that a molecular pathway may have different effects in different cellular contexts; the same pathway can be involved in different functions in different species or in different parts of an individual (Gilbert and Sarkar 2000, Laubichler and Wagner 2001, Burian  2004).[7]  These different contexts include developmental and physiological history, which condition the behavior of molecular pathways (Brigandt 2006, Kirschner 2005). Even the amino acid sequence produced by a molecular gene may depend on both DNA elements outside of the gene and non-genetic factors, which means the same gene can code for distinct products in different cells or different states of a cell (Griffiths and Stotz 2013, Stotz 2006a,b). Thus, there are one-many relations between molecular kinds and higher level kinds: a molecular mechanism can causally lead to or be part of different higher level states depending on the  context.[8] 