Analyses of gene expression profiles have shown that tumours arising in young women can be distinguished by 367 biologically relevant gene sets16 and that ER-positive tumours in premenopausal women overexpress AREG, TFPI2, AMPH, DBX2, RP5–1054A22.3 and KLK5, and underexpress ESR1, CYP4Z1, RANBP3L, FOXD2 and PEX317,18. The gene discussed is KLK5; the disease is neoplasm.