Treg cells are found in higher numbers in invasive breast cancers than in-situ breast cancers, in ER negative more so than ER positive breast tumors and are associated with higher rates of relapse and poorer survival.74,75 Treg cell infiltration may be useful as a prognostic marker for identifying patients with a favorable benefit-to-risk ratio for undergoing further hormonal therapy beyond the standard dose. This evidence concerns the gene ESR1 and breast neoplasm.