Early studies of CD39+CD8+ T cells investigated the comparison with their counterpart, CD39−CD8+ T cells, in TILs or metastatic lymph nodes from 33 untreated patients with breast cancer and 4 patients with melanoma, as well as in mouse tumor models (breast cancer and melanoma), the former expressed relatively low levels of TNF-α, IFN-γ and IL-2, and were negatively associated with the secretion of granzyme B and perforin. This evidence concerns the gene PRF1 and breast cancer.