Correlation between Ox-E/ER expression index and clinical parameters in a set of 394 ER-positive breast tumors. (a) Scatter-plot of progesterone receptor (PR) expression levels versus Ox-E/ER index values. Line denotes the linear regression fit of the Ox-E/ER index to PR gene expression. (b) Scatter-plot of age versus Ox-E/ER index. Line denotes the linear regression fit of the index to age. (c) Box plot of the Ox-E/ER index versus tumor grade. Significant differences in Ox-E/ER index with respect to grade are denoted with symbols (# and †; t-test, P < 0.05). Pearson correlations between the index and grade are also shown. (d) Box plot of the Ox-E/ER index versus ERBB2 tumor status, determined as described in Materials and methods. Pearson correlations between the index and ERBB2 status are also shown. *Statistically significant differences in Ox-E/ER index with ERBB2 status (t-test, P < 0.05). ER, estrogen receptor; Ox-E/ER, oxidant-sensitive estrogen/ER gene signature.