Hypoxia-inducible factor-1alpha (HIF1α), an Hsp90 client important for the angiogenesis of tumour growth, is partly protected from Hsp90 inhibitor-mediated degradation in COS cells when—in the form of a heterodimer with aryl hydrocarbon receptor nuclear translocator (ARNT)—it constitutes a functional transcription factor [90]. Here, ARNT is linked to neoplasm.