Profiling by qRT-PCR of ER-positive/FGFR1-amplified IDCs, deprived of estrogen and treated with FGF3/FGF19, identified a subset of ER-responsive genes, which included TFF1, CCND1, THSB1, CTGF, CCL2 and EGR3. In addition, gene-set enrichment analysis (GSEA) from letrozole-treated IDC patients revealed, that ER-related pathways were still active in ER-positive/FGFR1-amplified primary tumours. This evidence concerns the gene ESR1 and neoplasm.