Students are frequently overwhelmed by the complexity of metabolic pathways and they think they have "learned" the pathway when they have memorized the individual reactions.  This laboratory lesson helps students to understand the significance of individual reactions in the pathways leading to methionine synthesis in the budding yeast, Saccharomyces cerevisiae.  Students appreciate that methionine is one of only two sulfur-containing amino acids, and students do not find it difficult to follow the "yellow" sulfur atom in the pathway. In the lesson, students use three different yeast met strains, each of which lacks a single gene involved in methionine synthesis.  Working in groups of three, students identify the missing MET gene in each of the three deletion strains by analyzing the abilities of the deletion strains to grow on several defined media in which methionine has been replaced with alternative sulfur sources. Students also determine the position of mutant genes in the pathway relative to sulfite reductase, using indicator media that reacts with sulfide, the product of the reaction catalyzed by sulfite reductase. For the analysis, students prepare serial dilutions of yeast cultures and spot the dilution series on agar plates. This lesson is part of a semester-long research investigation into the evolutionary conservation of the genes involved in methionine synthesis. The lesson can also be used as a stand-alone exercise that teaches students about biochemical pathways, while reinforcing basic microbiological techniques.