Classical homocystinuria (HCU) is the most common loss-of-function inbornerror of sulfur amino acid metabolism. HCU is caused by a deficiency in enzymaticdegradation of homocysteine, a toxic intermediate of methionine transformationto cysteine, chiefly due to missense mutations in the cystathionine betasynthase(CBS) gene. As with many other inherited disorders, the pathogenicmutations do not target key catalytic residues, but rather introduce structuralperturbations leading to an enhanced tendency of the mutant CBS to misfoldand either to form nonfunctional aggregates or to undergo proteasome-dependentdegradation. Correction of CBS misfolding would represent an alternative therapeuticapproach for HCU. In this review, we summarize the complex nature ofCBS, its multi-domain architecture, the interplay between the three cofactorsrequired for CBS function [heme, pyridoxal-50-phosphate (PLP), andS-adenosylmethionine (SAM)], as well as the intricate allosteric regulatorymechanism only recently understood, thanks to advances in CBS crystallography.While roughly half of the patients respond to treatment with a PLP precursorpyridoxine, many studies suggested usefulness of small chemicals, such aschemical and pharmacological chaperones or proteasome inhibitors, rescuingmutant CBS activity in cellular and animal models of HCU. Non-specific chemicalchaperones and proteasome inhibitors assist in mutant CBS folding processand/or prevent its rapid degradation, thus resulting in increased steady-state levelsof the enzyme and CBS activity. Recent interest in the field and availablestructural information will hopefully yield CBS-specific compounds, by usinghigh-throughput screening and computational modeling of novel ligands, improvingfolding, stability, and activity of CBS mutants.