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Exposure of cells or tissues of various living organisms to elevated temperatures induces the synthesis of a family of specific proteins called heat-shock proteins (HSPs)^1–3. This phenomenon has so far been investigated mostly with respect to the induction mechanism of these HSPs^4,5. However, little is known about the function of such proteins, although it has been suggested that they are involved in the acquisition of thermal tolerance³. We have recently suggested that a specific class of HSPs may function as negatively regulatory molecules in the growth of eukaryotic cells^6–8, and that a HSP of relative molecular mass (M_r) 48,000 (HSP48) from the yeast Saccharomyces cerevisiae may be involved in both thermal tolerance and growth control in this organism⁸. We now present evidence that S. cerevisiae HSP48 is an isoprotein of a glycolytic enzyme, enolase (EC 4.2.1.11). This unexpected finding may provide …