Among the 20 cancer-relevant mutants examined, S463P/D538G showed the highest levels of transcriptional activity in the absence of DES (133×basal ERα-wt levels), which is followed by Y537N/D538G and Y537S/D538G (108×and 95×basal ERα-wt levels), and single mutant Y537S, D538G, and Y537N (79×, 24×, and 5×basal ERα-wt levels). Here, ESR1 is linked to cancer.