We applied quantitative immunofluorescence histocytometry to correlate levels of PD-L2 on malignant epithelial and stromal cells with clinical outcome in luminal breast cancer in a main patient cohort treated at one medical center (n = 684) and in an independent validation cohort (n = 273) from a separate medical center. The gene discussed is PDCD1LG2; the disease is breast carcinoma.