ESR1 and breast cancer: This analysis shows that ESR1 and PGR expression demonstrate a highly asymmetric relationship, in which PGR expression tends to be low/absent in ER- breast cancer, with >95% of ER- cases showing relatively low-levels of PGR expression (less than the cut-point of 0.4), while PGR expression varies from low-to-high in ER+ breast cancer, with 43% of ER+ breast cancers showing relatively high levels of PGR expression and 57% of ER+ breast cancers showing relatively low levels (Figure 2B).