STS and breast cancer: In ER-positive (ER+) breast cancer, which accounts for ∼70% of all breast cancers,4 oestrogens promote tumour growth.5 Approximately ten times more oestrone (E1) is derived from oestrone sulfate (E1S) via STS than that from androstenedione via aromatase.6,7 Therefore, the simultaneous dual inhibition of oestrogen synthesis from androgens through aromatase and oestrogen sulfates (E1S and E2S) through STS (Fig. 1) was an interesting approach, leading to the development of several dual aromatase/STS inhibitors (DASIs).8,9