The 17q23 region is amplified in several breast cancer cell lines and in 10–30% of primary tumors, resulting in S6K1 overexpression.28,29 We have previously shown that estrogen-activated ERα positively regulates S6K1 expression by a GATA-3-dependent mechanism in mammary epithelia.30 ERα regulation of S6K1 expression creates a positive feedforward loop, leading to activation of ERα by S6K1, potentiating ERα-positive breast cancer cell growth. Here, ESR1 is linked to breast cancer.