ESR1 and breast neoplasm: Although ERα expression alone was sufficient to promote PgR expression in breast tumors, the levels of PgR expression were significantly potentiated in tumors coexpressing ERα with nuclear 4ICD (PgR AQUA 21.7 vs. 30.0; p = 0.025) indicating that the 4ICD coactivator potentiates PgR expression in breast tumors.