A semester-long hypothesis-driven laboratory project for second-semester biochemistry has been developed. Working independently, students propose a hypothesis about the role of one amino acid residue in the active site of 5,10-methenyltetrahydrofolate synthetase (MTHFS). They then test this hypothesis by site-directed mutagenesis of a plasmid encoding the wild-type enzyme, overexpression and purification of the mutant protein, and detailed kinetic characterization. Student success in this laboratory course requires attention to detail and careful preservation of materials from week to week. Controls are carefully selected to allow diagnosis of experimental failures, and students are required to discuss them in their laboratory reports. The combination of independent work coupled with experiments in which results from one week influence the next provides students with a model of professional research activities that is absent from traditional laboratory experiments.