The wild-type ERG19 gene of the yeast Saccharomyces cerevisiae encoding mevalonate diphosphate decarboxylase (MVD) and the mutated recessive erg19-34 allele leading to a decrease of sterol production and to a thermosensitive phenotype have been characterized [2]. The mutated erg19-34 allele bears a single amino acid leucine79-to-proline (L79P) substitution. It was shown that this mutation does not affect the level of production of the enzyme. We performed a two-hybrid assay to show that the yeast Saccharomyces cerevisiae MVD forms homodimers in vivo and that the single point mutation drastically impairs the oligomerization of the protein, thereby explaining the deficiency of MVD activity observed in the temperature-sensitive strain.