ESR1 and breast cancer: In a population-based breast cancer series (Series 1, n = 518), strong and consistent associations were found between PRSS2 expression and multiple features of aggressive tumors, such as high histologic grade, lack of expression of estrogen receptor (ER), tumor cell proliferation (by Ki67 expression), CK5/6 expression (basal marker), and increased angiogenesis (by pMVD and GMP) (Table 1).