Integrative cluster 4 englobes both ER+ and ER- breast cancers, including 26% of TNBCs and basal-like tumors; at the molecular level, these tumors have low genomic instability and a low level of CNAs; at the mutational level, the most frequent mutations are at the level of PIK3CA (28%) and TP53 (20%). The gene discussed is ESR1; the disease is breast cancer.