Heat shock protein 90α (Hsp90α), encoded by the HSP90AA1 gene, is the stress inducible isoform of the molecular chaperone Hsp90. Hsp90α is regulated differently and has different functions when compared to the constitutively expressed Hsp90β isoform, despite high amino acid sequence identity between the two proteins. These differences are likely due to variations in nucleotide sequence within non-coding regions, which allows for specific regulation through interaction with particular transcription factors, and to subtle changes in amino acid sequence that allow for unique post-translational modifications. This article will specifically focus on the expression, function and regulation of Hsp90α.