Upon receiving his Ph.D. in 1947, he was immediately hired as an assistant professor in physics at Purdue. However, Benzer was inspired by Erwin Schrödinger's book What Is Life?, in which the physicist pondered the physical nature of the gene and a “code” of life. This catalyzed Benzer's shift in interest to biology, and he moved into the area of bacteriophage genetics., spending two years as a postdoctoral fellow in Max Delbrück 's laboratory at California Institute of Technology, and then returning to Purdue. At Purdue University, Benzer developed the T4 rII system, a new genetic technique involving recombination in T4 bacteriophage rII mutants. After observing that a particular rII mutant, a mutation that caused the bacteriophage to eliminate bacteria more rapidly than usual, was not exhibiting the expected phenotype, it occurred to Benzer that this strain might have come from a cross between two different rII mutants (each having part of the rII gene intact) wherein a recombination event resulted in a normal rII sequence. Benzer realized that by generating many r mutants and recording the recombination frequency between different r strains, one could create a detailed map of the gene, much as Alfred Sturtevant had done for chromosomes. Taking advantage of the enormous number of recombinants that could be analyzed in the rII mutant system, Benzer was eventually able to map over 2400 rII mutations. The data he collected provided the first evidence that the gene is not an indivisible entity, as previously believed, and that genes were linear. Benzer also proved that mutations were distributed in many different parts of a single gene, and the resolving power of his system allowed him to discern mutants that differ at the level of a single nucleotide. Based on his rII data, Benzer also proposed distinct classes of mutations including deletions, point mutations, missense mutations, and nonsense mutations.