Increased STS activity has been demonstrated in some hormone-dependent breast cancers: The enzyme cleaves sulfate from biologically inactive estrogen-3-sulfates (estrone-3-sulfate, estradiol-3-sulfate, or estriol-3-sulfate) to release the desulfonated estrogen (see Figure 9), which in turn increases tumor growth by activating the estrogen receptor. The gene discussed is ESR1; the disease is breast cancer.