 Along with Brenner, in the late 1960s and early 1970s, many of the leading molecular biologists from the classical period redirected their research agendas, utilizing the newly developed molecular techniques to investigate unsolved problems in other fields. Francois Jacob, Jacques Monod and their colleagues used the bacteria Escherichia coli to investigate how environmental conditions impact gene expression and regulation (Jacob and Monod 1961; discussed in Craver and Darden 2013; Morange 1998: Ch. 14; Schaffner 1974a; Weber 2005; see also the entry on the  developmental biology).  The study of behavior and the nervous system also lured some molecular biologists. Finding appropriate model organisms that could be subjected to molecular genetic analyses proved challenging. Returning to the fruit flies used in Mendelian genetics, Seymour Benzer induced behavioral mutations in Drosophila as a “genetic scalpel” to investigate the pathways from genes to behavior (Benzer 1968; Weiner 1999). And at Cambridge, Sydney Brenner developed the nematode worm, Caenorhabditis elegans, to study the nervous system, as well as the genetics of behavior (Brenner 1973, 2001; Ankeny 2000; Brown 2003). In subsequent decades, the study of cells was transformed from descriptive cytology into molecular cell biology (Alberts et al. 1983; Alberts et al. 2002; Bechtel 2006). Molecular evolution developed as a phylogenetic method for the comparison of DNA sequences and whole genomes; molecular systematics sought to research the evolution of the genetic code as well as the rates of that evolutionary process by comparing similarities and differences between molecules (Dietrich 1998; see also the entries on  evolution,  heritability, and  adaptationism).  The immunological relationship between antibodies and antigens was recharacterized at the molecular level (Podolsky and Tauber 1997; Schaffner 1993; see also the entry on the  philosophy of immunology).  And the study of oncogenes in cancer research as well as the molecular bases of mental illness were examples of advances in molecular medicine (Morange 1997b; see also the entry on  philosophy of psychiatry).