In our initial clinical study of FFNP-PET in breast cancer patients (all of whom were postmenopausal) (NCT00968409), we found that FFNP uptake was greater in PgR-positive than PgR-negative breast cancers (Figure 20) and the average SUV values were separated; uptake values, however, were neither very high nor markedly separated [4], very likely because of the low levels of circulating estradiol in these women. Here, PGR is linked to breast cancer.