PGR and breast cancer: Up to 29% of ER+ breast cancers have a heterozygous or homozygous deletion of PGR, which occurs more often in the luminal B subtype of breast cancers.12 Therefore, it is not surprising that the luminal B breast cancer subtype is associated with a higher proliferation rate and poorer prognosis than luminal A cancers.16 Loss of PGR is a mechanism by which ER+ tumours may evade the antagonistic effect of PR signalling on ER-mediated oncogenesis.