With respect to this, the naturally occurring flavonoid quercetin (3,3′,4′,5,7-pentahydroxyflavone) is one of the first Hsp70 inhibitors that was shown to reduce the expression of Hsp70 by the inhibition of HSF1 phosphorylation and transcriptional activity, and to sensitize tumour cells to apoptosis induced by anti-cancer drugs by inhibiting Hsp70 and Mcl-1 expression, while activating pro-apoptic Bax [254-257]. This evidence concerns the gene HSF1 and cancer.