 Without traditional laws of nature from which to derive explanations, philosophers of biology have been forced to rethink the nature of scientific explanation in biology and, in particular, molecular biology. Two accounts of explanation emerged: the unificationist and the causal-mechanical. Philip Kitcher (1989, 1993) developed a unificationist account of explanation, and he and Sylvia Culp explicitly applied it to molecular biology (Culp and Kitcher 1989). Among the premises of the “Watson-Crick” argument schema were “transcription, post-transcriptional modification and translation for the alleles in question”, along with details of cell biology and embryology for the organisms in question (Kitcher 1989). An explanation of a particular pattern of distribution of progeny phenotypes in a genetic cross resulted from instantiating the appropriate deductive argument schema: the variables were filled with the details from the particular case and the conclusion derived from the premises.